## What changed
- Add a feature-gated `TurnSettings` operation that can update the model,
reasoning effort, reasoning summary, and service tier for subsequent steps of
a named running turn without changing future thread settings.
- Report whether an update was applied, rejected, or lost its live target.
- Revalidate managed constraints and preserve the turn's admitted approval and
Guardian safety properties before publishing an updated settings snapshot.
## Testing
- Cover sparse updates, step capture and ordering, target replacement, managed
policy changes, model metadata safety, and separation from future settings.
GitOrigin-RevId: ecaaaa95b4fd4ec9d62265bef551582dddb36d78
