Fix WebSocketManager to reset broadcast task after cancellation

Author: Dylan Huang

eval_protocol/utils/logs_server.py

@@ -109,6 +109,7 @@ def stop_broadcast_loop(self):
         """Stop the broadcast loop."""
         if self._broadcast_task and not self._broadcast_task.done():
             self._broadcast_task.cancel()
+            self._broadcast_task = None
 
 
 class EvaluationWatcher: