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/features/src/lib.rs

@@ -309,6 +309,8 @@ pub enum Feature {
     Artifact,
     /// Enable Fast mode selection in the TUI and request layer.
     FastMode,
+    /// Enable explicitly requested model changes for later step captures.
+    StepModelSwitching,
     /// Enable experimental realtime voice conversation mode in the TUI.
     RealtimeConversation,
     /// Prevent idle system sleep while a turn is actively running.
@@ -1515,6 +1517,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::StepModelSwitching,
+        key: "step_model_switching",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::RealtimeConversation,
         key: "realtime_conversation",