feature: Add support GetHeader() Method in SSE Transport Sessions (ClientSession) #443
Description
Problem Statement
The MCP-Go library's SSE (Server-Sent Events) transport was missing the ability to access HTTP headers from client requests. While the ClientSession interface defined a GetHeader() method that should return http.Header, none of the session implementations actually implemented this method, causing compilation errors and preventing developers from accessing important request metadata like authentication headers, user agents, or custom headers.