Fix missing imports in MCP documentation examples

[MariumAfzal2703]

This PR fixes missing imports in the MCP documentation (mcp.md).
Several code snippets use Agent, Runner, ModelSettings, or MCPServer* classes without including the corresponding import statements. This makes the examples fail with NameError when copied and run directly.

Changes included

• Added missing Agent, Runner, and ModelSettings imports in the SSE MCP servers example.
• Added missing Agent, Runner, and MCPServerStdio imports in the Stdio MCP servers example.
• Added missing Agent import in the Prompts example.
• Ensured all snippets are runnable out of the box with consistent imports.

Why this matters

• Improves developer experience by making examples copy-paste runnable.
• Prevents confusion for new users who encounter NameError.
• Keeps documentation consistent and complete.

[seratch]

Thanks for improving this!