Validate and persist API chat lifetime: lifetime_hours is validated as a positive number and stored in the AgentContext data, and context.last_message is set using UTC. Removed the in-class threading-based cleanup state and old _cleanup_expired_chats method. Introduced a new job-loop extension (extensions/python/job_loop/_20_cleanup_expired_api_chats.py) that periodically scans AgentContext instances and removes expired API chats (using persist_chat.remove_chat) in a UTC-aware manner. Added tests (tests/test_api_chat_lifetime.py) to verify lifetime persistence and that the job loop removes expired chats.
