fix: close SSE stream writers in terminate() to allow graceful completion

Author: kaushik701

src/mcp/server/streamable_http.py

@@ -771,6 +771,11 @@ async def terminate(self) -> None:
         self._terminated = True
         logger.info(f"Terminating session: {self.mcp_session_id}")
 
+        # Close all SSE stream writers to allow EventSourceResponse to complete gracefully
+        for writer in list(self._sse_stream_writers.values()):
+            writer.close()
+        self._sse_stream_writers.clear()
+
         # We need a copy of the keys to avoid modification during iteration
         request_stream_keys = list(self._request_streams.keys())