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/src/session/mod.rs

@@ -3479,11 +3479,14 @@ impl Session {
             if settings.selected().service_tier != root_service_tier {
                 let mut selected = settings.selected().clone();
                 selected.service_tier = root_service_tier;
-                settings = Arc::new(ResolvedStepSettings::new(
+                let mut inherited_settings = ResolvedStepSettings::new(
                     Arc::new(selected),
                     Arc::clone(&settings.model_info),
                     self.features.enabled(Feature::FastMode),
-                ));
+                );
+                inherited_settings.mcp_approvals_reviewer_override =
+                    settings.mcp_approvals_reviewer_override;
+                settings = Arc::new(inherited_settings);
             }
         }
         let token_budget = token_budget::resolve_token_budget(