there is no import TaskCallback

### What happened?

Traceback (most recent call last): File "/Users/ninebot/Desktop/workspace/A2A/a2a-samples/demo/ui/main.py", line 28, in from service.server.server import ConversationServer File "/Users/ninebot/Desktop/workspace/A2A/a2a-samples/demo/ui/service/server/server.py", line 27, in from .adk_host_manager import ADKHostManager, get_message_id File "/Users/ninebot/Desktop/workspace/A2A/a2a-samples/demo/ui/service/server/adk_host_manager.py", line 35, in from hosts.multiagent.remote_agent_connection import ( TaskCallbackArg, ) ImportError: cannot import name 'TaskCallbackArg' from 'hosts.multiagent.remote_agent_connection' (/Users/ninebot/Desktop/workspace/A2A/a2a-samples/samples/python/hosts/multiagent/remote_agent_connection.py) (a2a-

### Relevant log output

```shell

```

### Code of Conduct

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

该错误是从 'hosts.multiagent.remote_agent_connection' 导入 'TaskCallbackArg' 时的 ImportError。查看文件 /samples/python/hosts/multiagent/remote_agent_connection.py，看看导出了什么。检查 TaskCallbackArg 是否在那里定义，或者是否需要从其他模块导入。修复很可能涉及添加缺失的 import，或更正该文件中的导出列表。在 demo/ui/main.py 中运行失败的 import，以验证修复。

由索引模型根据 Issue 内容生成。