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

codex-rs/core/src/session/mod.rs

@@ -229,6 +229,7 @@ mod rollout_budget;
 mod rollout_reconstruction;
 #[allow(clippy::module_inception)]
 pub(crate) mod session;
+mod step_activation;
 pub(crate) mod step_context;
 pub(crate) mod step_settings;
 mod thread_settings;