Rename approvals reviewer variant to auto-review (#19056)

## Why

`approvals_reviewer` now uses `auto_review` as the canonical config/API
value after #18504, but the Rust enum variant and nearby helper/test
names still used `GuardianSubagent` / guardian approval wording. That
made follow-up code and reviews confusing even though the external value
had already moved to Auto-review.

## What changed

- Renamed `ApprovalsReviewer::GuardianSubagent` to
`ApprovalsReviewer::AutoReview`.
- Updated protocol, app-server, config, core, TUI, exec, and analytics
test callsites.
- Renamed nearby helper/test names from guardian approval wording to
Auto-review wording where they refer to the approvals reviewer mode.
- Preserved wire compatibility:
  - `auto_review` remains the canonical serialized value.
  - `guardian_subagent` remains accepted as a legacy alias.

This intentionally does not rename the `[features].guardian_approval`
key, `Feature::GuardianApproval`, `core/src/guardian`, analytics event
names, or app-server Guardian review event types.

## Verification

- `cargo test -p codex-protocol
approvals_reviewer_serializes_auto_review_and_accepts_legacy_guardian_subagent`
- `cargo test -p codex-app-server-protocol
approvals_reviewer_serializes_auto_review_and_accepts_legacy_guardian_subagent`
- `cargo test -p codex-config approvals_reviewer`
- `cargo test -p codex-tui update_feature_flags`
- `cargo test -p codex-core permissions_instructions`
- `cargo test -p codex-tui permissions_selection`

codex-rs/core/src/guardian/review.rs

@@ -146,7 +146,7 @@ pub(crate) fn routes_approval_to_guardian(turn: &TurnContext) -> bool {
     matches!(
         turn.approval_policy.value(),
         AskForApproval::OnRequest | AskForApproval::Granular(_)
-    ) && turn.config.approvals_reviewer == ApprovalsReviewer::GuardianSubagent
+    ) && turn.config.approvals_reviewer == ApprovalsReviewer::AutoReview
 }
 
 pub(crate) fn is_guardian_reviewer_source(

codex-rs/core/src/guardian/tests.rs

@@ -909,7 +909,7 @@ async fn routes_approval_to_guardian_requires_guardian_reviewer() {
 
     assert!(!routes_approval_to_guardian(&turn));
 
-    config.approvals_reviewer = ApprovalsReviewer::GuardianSubagent;
+    config.approvals_reviewer = ApprovalsReviewer::AutoReview;
     turn.config = Arc::new(config);
 
     assert!(routes_approval_to_guardian(&turn));
@@ -919,7 +919,7 @@ async fn routes_approval_to_guardian_requires_guardian_reviewer() {
 async fn routes_approval_to_guardian_allows_granular_review_policy() {
     let (_session, mut turn) = crate::session::tests::make_session_and_context().await;
     let mut config = (*turn.config).clone();
-    config.approvals_reviewer = ApprovalsReviewer::GuardianSubagent;
+    config.approvals_reviewer = ApprovalsReviewer::AutoReview;
     turn.config = Arc::new(config);
     turn.approval_policy
         .set(AskForApproval::Granular(GranularApprovalConfig {