stream bug

### What happened?

The server-side has already finished streaming the output, but the client-side only starts streaming output afterwards, and it prints everything out very quickly.

server code:
async for event in self.agent.stream(query):
message = TaskArtifactUpdateEvent(
context_id=context.context_id, # type: ignore
task_id=context.task_id, # type: ignore
artifact=new_text_artifact(
name='current_result',
text=event['content'],
),
)
await event_queue.enqueue_event(message)
print(event['content'])
if event['done']:
break

client_code ：
request = create_text_message_object(content=user_input)

# Send the request and get the streaming messages
async for response in client.send_message(request):
task, _ = response
print(get_message_text(task.artifacts[-1]))

### Relevant log output

```shell

```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Look at the server-side streaming loop in the agent.stream method and the client-side async for loop in client.send_message. The issue is about timing mismatch in streaming events. Start by examining the event queue implementation and the TaskArtifactUpdateEvent structure. Check if there's buffering or synchronization delay. Run the provided code snippets to reproduce the bug and see the print statements' order.

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