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/app-server-protocol/src/protocol/v2/turn.rs

@@ -47,6 +47,10 @@ pub enum TurnStatus {
 pub struct TurnSettingsUpdateParams {
     pub thread_id: String,
     pub turn_id: String,
+    /// Changes the active turn's reviewer without changing future thread settings.
+    /// Already captured steps and pending approvals retain their original reviewer.
+    #[ts(optional = nullable)]
+    pub approvals_reviewer: Option<ApprovalsReviewer>,
     /// Omission or `null` leaves the model unchanged.
     #[ts(optional = nullable)]
     pub model: Option<String>,