Fix StreamableHTTP SSE premature stream closure for server-initiated requests

The SSE writer was closing the stream immediately upon receiving ANY
JSONRPCResponse, including responses to server-initiated requests like
elicitation. This prevented bidirectional communication patterns from
working correctly over StreamableHTTP transport.

Changed the SSE writer to only close the stream when receiving the
response to the ORIGINAL client request, allowing server-initiated
request/response cycles (elicitation, sampling, etc.) to complete
before closing the stream.

Impact:
- Elicitation now works correctly over StreamableHTTP transport
- Sampling and other server-initiated requests will also work
- No breaking changes to existing functionality

Testing:
- Manually verified elicitation request/response cycle completes
- Server logs show: request sent → response received → tool completes
- Existing tests continue to pass

Author: felixweinberger

src/mcp/server/streamable_http.py

@@ -467,12 +467,16 @@ async def sse_writer():
                                 event_data = self._create_event_data(event_message)
                                 await sse_stream_writer.send(event_data)
 
-                                # If response, remove from pending streams and close
+                                # Only close when we receive the response to the ORIGINAL request
+                                # Server-initiated requests (like elicitation) will have different IDs
                                 if isinstance(
                                     event_message.message.root,
                                     JSONRPCResponse | JSONRPCError,
                                 ):
-                                    break
+                                    response_id = str(event_message.message.root.id)
+                                    if response_id == request_id:
+                                        break
+                                    # Otherwise, continue streaming - this is a response to a server-initiated request
                     except Exception:
                         logger.exception("Error in SSE writer")
                     finally: