## Why
A recovery turn can occupy the core turn slot before `TurnStarted` is published and thread statuses reflect it. During this window, `thread/resume` with configuration overrides could treat an unsubscribed thread as idle and shut it down, interrupting recovery.
## What changed
Require the core turn slot to be empty before replacing a loaded thread during resume. Expose `CodexThread::has_active_turn()` and reuse it in the residency unload check.
## Testing
Add a regression test that blocks paginated history writes to hold a recovery turn before status publication, reconnects with a configuration override, and verifies that the client receives completion for the preserved recovery turn.
GitOrigin-RevId: f295038c8502a9b2849ddc08614db4a3cb5610cf
Anton Panasenko
b75ec48a2c