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/protocol/src/protocol.rs

@@ -490,6 +490,8 @@ pub struct ConversationSpeechParams {
 /// task kind. Child sessions and consumers of frozen initial settings are unchanged.
 #[derive(Debug, Clone, Default, PartialEq, Eq)]
 pub struct TurnSettingsUpdate {
+    /// Changes the reviewer for subsequent approval requests, not pending reviews.
+    pub approvals_reviewer: Option<ApprovalsReviewer>,
     pub model: Option<String>,
     /// `None` preserves the selection; `Some(None)` clears it.
     pub effort: Option<Option<ReasoningEffortConfig>>,