ManageApiClient 单例与守护线程的竞态条件仍需修复

Background

PR #3336 has added if not ManageApiClient._instance: return None to the guard thread entry points (generate_and_save_chat_summary/generate_and_save_chat_title), which mitigates the NoneType crash when the service is closed. However, this only narrows the window, and does not eliminate the race condition from the root.