fix(mothership): serialize headless run admission with chat soft delete - #9107

Summary

• Headless run admission (createRunSegment: task wake, inbox, v2 chat, Slack, knowledge chat, benchmarks) neither locked nor checked the chat row. A run inserted after a soft delete's live-run query was never stopped by that delete. Chat-view admission (admitChatTurn) was already safe because it UPDATEs the chat with deletedAt IS NULL.
• insertRunSegment now reads the chat row FOR SHARE, after the run-admission advisory lock, which is the same order admitChatTurn uses. FOR SHARE conflicts with the soft delete's UPDATE, so either the delete commits first and the run is inserted cancelled with tool admission closed at deletedAt, or the delete waits and its stopLiveRunsTx sees the run and stops it.
• Callers that use createRunSegment directly already treat a cancelled admission like a Stop that arrived before admission, without calling the worker. Slack was the exception: it ignored the status. It now ends the turn as that Stop would, so it never calls the worker.
• Hard delete was already serialized, because the run insert's FK KEY SHARE lock conflicts with the delete's FOR UPDATE. Behaviour for live chats is unchanged. The cost is one extra indexed primary-key read per run-segment insert.

Type of Change

• Bug fix

Testing

• removal-stops-runs.integration.ts (real Postgres and Redis):
  • a headless run admitted after the delete commits starts stopped;
  • a race between headless admission and the delete, forced in both orders behind a held chat row lock: either the delete's Stop is pushed and recorded, or the run is never live;
  • a headless run on a live chat stays live.
• assistant.integration.ts: a Slack turn whose chat was deleted before admission never starts the worker.
• The new tests fail on the previous code and pass with this change.

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)