Apply suggestion from @Tehsmash

Co-authored-by: Sam Betts <1769706+Tehsmash@users.noreply.github.com>

Author: muscariello

src/a2a/server/request_handlers/default_request_handler.py

@@ -536,7 +536,7 @@ async def on_subscribe_to_task(
         self,
         params: SubscribeToTaskRequest,
         context: ServerCallContext | None = None,
-    ) -> AsyncGenerator[Event]:
+    ) -> AsyncGenerator[StreamResponse]:
         """Default handler for 'SubscribeToTask'.
 
         Allows a client to re-attach to a running streaming task's event stream.