fix: gracefully terminate active StreamableHTTP sessions during shutdown

Author: kaushik701

src/mcp/server/streamable_http_manager.py

@@ -133,6 +133,12 @@ async def lifespan(app: Starlette) -> AsyncIterator[None]:
                 yield  # Let the application run
             finally:
                 logger.info("StreamableHTTP session manager shutting down")
+                # Gracefully terminate all active sessions before cancelling tasks
+                for transport in list(self._server_instances.values()):
+                    try:
+                        await transport.terminate()
+                    except Exception:
+                        logger.debug("Error terminating transport during shutdown", exc_info=True)
                 # Cancel task group to stop all spawned tasks
                 tg.cancel_scope.cancel()
                 self._task_group = None