[ADD] ai_connection_queue

[angelmoya]

Add ai_connection_queue: run ai.connection conversations via queue_job

• ai_connection: refactor _run_ai's while loop into _run_ai_step with
  _run_ai_next/_run_ai_finalize/_run_ai_error extension points. No
  behavior change.
• New module ai_connection_queue: dispatches each round through
  queue_job instead of blocking, opt-in via ai_connection_async
  context key. Returns an ai.connection.run id, not the final answer.
• Optional per-round notify callback (record-bound or model-level) via
  ai_connection_notify_model/_res_id/_method context keys.