Add live turn settings updates (#40653)

## 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