fix: clean up SSE session on client disconnect (#2200)

Co-authored-by: Varun Sharma <sharmava@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Max Isbey <224885523+maxisbey@users.noreply.github.com>

src/mcp/server/sse.py

@@ -186,6 +186,7 @@ class SseServerTransport:
                 )
                 await read_stream_writer.aclose()
                 await write_stream_reader.aclose()
+                self._read_stream_writers.pop(session_id, None)
                 logging.debug(f"Client session disconnected {session_id}")
 
             logger.debug("Starting SSE response task")