Implement SSE Transport Module

[MCPClaude]

Based on the documentation in docs/sse-transport.md, there's a comprehensive plan for implementing a dedicated SSE transport module that hasn't been fully implemented yet.

The document outlines a detailed approach including:

1. Adding SSE to TransportType enum
2. Creating src/transports/sseTransport.ts
3. Registering route in src/server.ts
4. Implementing heartbeat and ID tracking
5. Adding client polyfill documentation
6. Creating unit tests
7. Updating README badges/docs

It appears that some SSE functionality has been implemented in server.ts, but not as a dedicated transport module. The documentation provides a reference implementation that should be evaluated and implemented.

Recommended actions:

1. Review the current SSE implementation in server.ts
2. Implement the modular transport approach outlined in the documentation
3. Add appropriate tests for the new transport module
4. Update documentation to reflect the implementation