Queue Pending Message

Request model for queueing a follow-up while a turn is running.

Response returned when a message is queued because the session already has a turn in flight.

• buffer_length: how many messages are now in the session's pending buffer (after this push)
• max_buffer_length: the per-session cap (server-side constant)
• turn_in_flight: True if a copilot turn was running when we checked — purely informational for UX feedback.