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/session/tests.rs

@@ -3827,7 +3827,7 @@ async fn user_turn_updates_approvals_reviewer() {
             }],
             cwd: config.cwd.to_path_buf(),
             approval_policy: config.permissions.approval_policy.value(),
-            approvals_reviewer: Some(codex_config::types::ApprovalsReviewer::GuardianSubagent),
+            approvals_reviewer: Some(codex_config::types::ApprovalsReviewer::AutoReview),
             sandbox_policy: config.permissions.sandbox_policy.get().clone(),
             model: turn_context.model_info.slug.clone(),
             effort: config.model_reasoning_effort,
@@ -3843,7 +3843,7 @@ async fn user_turn_updates_approvals_reviewer() {
     let state = session.state.lock().await;
     assert_eq!(
         state.session_configuration.approvals_reviewer,
-        codex_config::types::ApprovalsReviewer::GuardianSubagent
+        codex_config::types::ApprovalsReviewer::AutoReview
     );
 }