Submit an asynchronous conversation turn

Atomically claim (caller, turn_ref). Canonical business digest excludes messages[].attachments[].signed_url. Identical replay returns the original ai_turn_id/session_id with replayed=true; changed business fields conflict. An aborted turn returns STATE.ABORTED. Superseding an unseen turn creates a 24-hour tombstone so late submission cannot run. Callback destination is registered per caller: Assistant POST /internal/assistant/v1/turn-results, mTLS caller kind ai; 200 {accepted:true}, unknown turn_ref 404 STATE.NOT_FOUND stops retries. Callback may precede submit response. Callback schema is defined only here.

POST /internal/ai/v1/conversation-turns