fix: gracefully handle task exceptions in event consumer

Author: onematchfox

src/a2a/server/events/event_consumer.py

@@ -160,5 +160,5 @@ def agent_task_callback(self, agent_task: asyncio.Task[None]) -> None:
             agent_task: The asyncio.Task that completed.
         """
         logger.debug('Agent task callback triggered.')
-        if agent_task.exception() is not None:
+        if not agent_task.cancelled and agent_task.done:
             self._exception = agent_task.exception()