Race conditions in SSE server code

It seems like there's data races (at least during testing):

```
==================
WARNING: DATA RACE
Read at 0x00c003564228 by goroutine 47:
  bufio.(*Writer).Available()
      /opt/homebrew/opt/go/libexec/src/bufio/bufio.go:656 +0xa8
  bufio.(*Writer).Write()
      /opt/homebrew/opt/go/libexec/src/bufio/bufio.go:674 +0x1c
  fmt.Fprintf()
      /opt/homebrew/opt/go/libexec/src/fmt/print.go:225 +0x94
  net/http.(*chunkWriter).Write()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:383 +0x2c0
  bufio.(*Writer).Flush()
      /opt/homebrew/opt/go/libexec/src/bufio/bufio.go:639 +0xc0
  net/http.(*response).FlushError()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:1772 +0x60
  net/http.(*response).Flush()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:1765 +0x28
  github.com/mark3labs/mcp-go/server.(*SSEServer).handleMessage()
      /Users/x/workspace/go/pkg/mod/github.com/mark3labs/mcp-go@v0.8.4/server/sse.go:196 +0x560
  github.com/mark3labs/mcp-go/server.(*SSEServer).handleMessage-fm()
      <autogenerated>:1 +0x4c
  net/http.HandlerFunc.ServeHTTP()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:2220 +0x48
  net/http.(*ServeMux).ServeHTTP()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:2747 +0x1c4
  net/http.serverHandler.ServeHTTP()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3210 +0x2a8
  net/http.(*conn).serve()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:2092 +0xe58
  net/http.(*Server).Serve.gowrap3()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3360 +0x4c

Previous write at 0x00c003564228 by goroutine 40:
  bufio.(*Writer).Flush()
      /opt/homebrew/opt/go/libexec/src/bufio/bufio.go:651 +0x2a8
  net/http.(*chunkWriter).flush()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:403 +0x8c
  net/http.(*response).FlushError()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:1773 +0x78
  net/http.(*response).Flush()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:1765 +0x28
  github.com/mark3labs/mcp-go/server.(*SSEServer).handleSSE()
      /Users/x/workspace/go/pkg/mod/github.com/mark3labs/mcp-go@v0.8.4/server/sse.go:149 +0x8ec
  github.com/mark3labs/mcp-go/server.(*SSEServer).handleSSE-fm()
      <autogenerated>:1 +0x4c
  net/http.HandlerFunc.ServeHTTP()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:2220 +0x48
  net/http.(*ServeMux).ServeHTTP()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:2747 +0x1c4
  net/http.serverHandler.ServeHTTP()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3210 +0x2a8
  net/http.(*conn).serve()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:2092 +0xe58
  net/http.(*Server).Serve.gowrap3()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3360 +0x4c

Goroutine 47 (running) created at:
  net/http.(*Server).Serve()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3360 +0x674
  net/http.(*Server).ListenAndServe()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3259 +0xb0
  github.com/mark3labs/mcp-go/server.(*SSEServer).Start()
      /Users/x/workspace/go/pkg/mod/github.com/mark3labs/mcp-go@v0.8.4/server/sse.go:74 +0x298
[snip]

Goroutine 40 (running) created at:
  net/http.(*Server).Serve()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3360 +0x674
  net/http.(*Server).ListenAndServe()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3259 +0xb0
  github.com/mark3labs/mcp-go/server.(*SSEServer).Start()
      /Users/x/workspace/go/pkg/mod/github.com/mark3labs/mcp-go@v0.8.4/server/sse.go:74 +0x298
[snip]
==================
==================
WARNING: DATA RACE
Write at 0x00c0035ac000 by goroutine 47:
  runtime.slicecopy()
      /opt/homebrew/opt/go/libexec/src/runtime/slice.go:355 +0x0
  bufio.(*Writer).Write()
      /opt/homebrew/opt/go/libexec/src/bufio/bufio.go:691 +0x2a8
  fmt.Fprintf()
      /opt/homebrew/opt/go/libexec/src/fmt/print.go:225 +0x94
  net/http.(*chunkWriter).Write()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:383 +0x2c0
  bufio.(*Writer).Flush()
      /opt/homebrew/opt/go/libexec/src/bufio/bufio.go:639 +0xc0
  net/http.(*response).FlushError()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:1772 +0x60
  net/http.(*response).Flush()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:1765 +0x28
  github.com/mark3labs/mcp-go/server.(*SSEServer).handleMessage()
      /Users/x/workspace/go/pkg/mod/github.com/mark3labs/mcp-go@v0.8.4/server/sse.go:196 +0x560
  github.com/mark3labs/mcp-go/server.(*SSEServer).handleMessage-fm()
      <autogenerated>:1 +0x4c
  net/http.HandlerFunc.ServeHTTP()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:2220 +0x48
  net/http.(*ServeMux).ServeHTTP()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:2747 +0x1c4
  net/http.serverHandler.ServeHTTP()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3210 +0x2a8
  net/http.(*conn).serve()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:2092 +0xe58
  net/http.(*Server).Serve.gowrap3()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3360 +0x4c

Previous read at 0x00c0035ac000 by goroutine 40:
  runtime.racereadrange()
      <autogenerated>:1 +0x14
  internal/poll.ignoringEINTRIO()
      /opt/homebrew/opt/go/libexec/src/internal/poll/fd_unix.go:745 +0x3f8
  internal/poll.(*FD).Write()
      /opt/homebrew/opt/go/libexec/src/internal/poll/fd_unix.go:381 +0x234
  net.(*netFD).Write()
      /opt/homebrew/opt/go/libexec/src/net/fd_posix.go:96 +0x44
  net.(*conn).Write()
      /opt/homebrew/opt/go/libexec/src/net/net.go:201 +0x84
  net.(*TCPConn).Write()
      <autogenerated>:1 +0x4c
  net/http.checkConnErrorWriter.Write()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3872 +0x60
  bufio.(*Writer).Flush()
      /opt/homebrew/opt/go/libexec/src/bufio/bufio.go:639 +0xc0
  net/http.(*chunkWriter).flush()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:403 +0x8c
  net/http.(*response).FlushError()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:1773 +0x78
  net/http.(*response).Flush()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:1765 +0x28
  github.com/mark3labs/mcp-go/server.(*SSEServer).handleSSE()
      /Users/x/workspace/go/pkg/mod/github.com/mark3labs/mcp-go@v0.8.4/server/sse.go:149 +0x8ec
  github.com/mark3labs/mcp-go/server.(*SSEServer).handleSSE-fm()
      <autogenerated>:1 +0x4c
  net/http.HandlerFunc.ServeHTTP()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:2220 +0x48
  net/http.(*ServeMux).ServeHTTP()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:2747 +0x1c4
  net/http.serverHandler.ServeHTTP()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3210 +0x2a8
  net/http.(*conn).serve()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:2092 +0xe58
  net/http.(*Server).Serve.gowrap3()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3360 +0x4c

Goroutine 47 (running) created at:
  net/http.(*Server).Serve()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3360 +0x674
  net/http.(*Server).ListenAndServe()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3259 +0xb0
  github.com/mark3labs/mcp-go/server.(*SSEServer).Start()
      /Users/x/workspace/go/pkg/mod/github.com/mark3labs/mcp-go@v0.8.4/server/sse.go:74 +0x298

Goroutine 40 (running) created at:
  net/http.(*Server).Serve()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3360 +0x674
  net/http.(*Server).ListenAndServe()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3259 +0xb0
  github.com/mark3labs/mcp-go/server.(*SSEServer).Start()
      /Users/x/workspace/go/pkg/mod/github.com/mark3labs/mcp-go@v0.8.4/server/sse.go:74 +0x298
==================
==================
WARNING: DATA RACE
Write at 0x00c0035ac116 by goroutine 47:
  runtime.slicecopy()
      /opt/homebrew/opt/go/libexec/src/runtime/slice.go:355 +0x0
  bufio.(*Writer).Write()
      /opt/homebrew/opt/go/libexec/src/bufio/bufio.go:691 +0x2a8
  net/http.(*chunkWriter).Write()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:391 +0x3a8
  bufio.(*Writer).Flush()
      /opt/homebrew/opt/go/libexec/src/bufio/bufio.go:639 +0xc0
  net/http.(*response).FlushError()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:1772 +0x60
  net/http.(*response).Flush()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:1765 +0x28
  github.com/mark3labs/mcp-go/server.(*SSEServer).handleMessage()
      /Users/x/workspace/go/pkg/mod/github.com/mark3labs/mcp-go@v0.8.4/server/sse.go:196 +0x560
  github.com/mark3labs/mcp-go/server.(*SSEServer).handleMessage-fm()
      <autogenerated>:1 +0x4c
  net/http.HandlerFunc.ServeHTTP()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:2220 +0x48
  net/http.(*ServeMux).ServeHTTP()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:2747 +0x1c4
  net/http.serverHandler.ServeHTTP()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3210 +0x2a8
  net/http.(*conn).serve()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:2092 +0xe58
  net/http.(*Server).Serve.gowrap3()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3360 +0x4c

Previous read at 0x00c0035ac110 by goroutine 40:
  runtime.racereadrange()
      <autogenerated>:1 +0x14
  internal/poll.ignoringEINTRIO()
      /opt/homebrew/opt/go/libexec/src/internal/poll/fd_unix.go:745 +0x3f8
  internal/poll.(*FD).Write()
      /opt/homebrew/opt/go/libexec/src/internal/poll/fd_unix.go:381 +0x234
  net.(*netFD).Write()
      /opt/homebrew/opt/go/libexec/src/net/fd_posix.go:96 +0x44
  net.(*conn).Write()
      /opt/homebrew/opt/go/libexec/src/net/net.go:201 +0x84
  net.(*TCPConn).Write()
      <autogenerated>:1 +0x4c
  net/http.checkConnErrorWriter.Write()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3872 +0x60
  bufio.(*Writer).Flush()
      /opt/homebrew/opt/go/libexec/src/bufio/bufio.go:639 +0xc0
  net/http.(*chunkWriter).flush()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:403 +0x8c
  net/http.(*response).FlushError()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:1773 +0x78
  net/http.(*response).Flush()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:1765 +0x28
  github.com/mark3labs/mcp-go/server.(*SSEServer).handleSSE()
      /Users/x/workspace/go/pkg/mod/github.com/mark3labs/mcp-go@v0.8.4/server/sse.go:149 +0x8ec
  github.com/mark3labs/mcp-go/server.(*SSEServer).handleSSE-fm()
      <autogenerated>:1 +0x4c
  net/http.HandlerFunc.ServeHTTP()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:2220 +0x48
  net/http.(*ServeMux).ServeHTTP()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:2747 +0x1c4
  net/http.serverHandler.ServeHTTP()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3210 +0x2a8
  net/http.(*conn).serve()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:2092 +0xe58
  net/http.(*Server).Serve.gowrap3()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3360 +0x4c

Goroutine 47 (running) created at:
  net/http.(*Server).Serve()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3360 +0x674
  net/http.(*Server).ListenAndServe()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3259 +0xb0
  github.com/mark3labs/mcp-go/server.(*SSEServer).Start()
      /Users/x/workspace/go/pkg/mod/github.com/mark3labs/mcp-go@v0.8.4/server/sse.go:74 +0x298
  [snip]

Goroutine 40 (running) created at:
  net/http.(*Server).Serve()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3360 +0x674
  net/http.(*Server).ListenAndServe()
      /opt/homebrew/opt/go/libexec/src/net/http/server.go:3259 +0xb0
  github.com/mark3labs/mcp-go/server.(*SSEServer).Start()
      /Users/x/workspace/go/pkg/mod/github.com/mark3labs/mcp-go@v0.8.4/server/sse.go:74 +0x298
[snip]
==================
```


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Race conditions in SSE server code #22

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Race conditions in SSE server code #22

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