Allow updating the approval reviewer for active turns (#42121)

## What changed

- Add `approvalsReviewer` to the experimental `turn/settings/update` API.
- Apply reviewer changes to subsequently captured steps and new background approval requests while preserving pending approvals and future-thread defaults.
- Allow reviewer-only updates without `step_model_switching`, while continuing to enforce managed reviewer restrictions and model-required auto review.
- Use an explicit live reviewer update for MCP approvals while retaining refreshed thread defaults for clients that have not set one.

## Testing

- Cover switching between user and automatic review during a live turn, future-turn isolation, and managed-authority rejection.

GitOrigin-RevId: e1216b09c6a7972c2c9eaf4955d8d57d60604ca1

codex-rs/core/tests/suite/step_settings.rs

@@ -247,6 +247,7 @@ async fn settings_updates_preserve_turn_identity_and_target(target: SettingsTarg
                 effort: Some(Some(ReasoningEffort::High)),
                 summary: Some(ReasoningSummary::Detailed),
                 service_tier: Some(Some(ServiceTier::Fast.request_value().to_string())),
+                ..Default::default()
             };
             assert_eq!(
                 submit_turn_settings(&test.codex, "different-turn", update.clone()).await?,