Bug fix: drain in-flight Feishu messages during shutdown

[fy17ohhh]

Fixes #901

Summary

Wire the AgentKit Feishu integration into FeishuChannelExtension's lifecycle so that in-flight messages can finish during application shutdown.

Previously, AgentKit maintained a separate background thread and called the wrapped SDK Channel's start() and stop() methods directly. This bypassed FeishuChannelExtension.start() and shutdown(), so message tasks were not drained when the process received SIGTERM.

Also rebinded lark_channel’s module-level WebSocket event loop to the dedicated Feishu worker loop before starting its synchronous client, preventing the this event loop is already running error message

Testing

uv run pre-commit run --all-files
uv run pytest tests/integrations/agentkit/test_app.py
uv run pytest tests/test_feishu_channel_extension.py

Behavior After This Change

The user can also receive the reply from Feishu bot with graceful shutdown now.