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/context/permissions_instructions.rs

@@ -188,7 +188,7 @@ fn approval_text(
         ),
     };
 
-    if approvals_reviewer == ApprovalsReviewer::GuardianSubagent
+    if approvals_reviewer == ApprovalsReviewer::AutoReview
         && approval_policy != AskForApproval::Never
     {
         format!("{text}\n\n{AUTO_REVIEW_APPROVAL_SUFFIX}")

codex-rs/core/src/context/permissions_instructions_tests.rs

@@ -197,10 +197,10 @@ fn on_request_includes_tool_guidance_alongside_inline_permission_guidance_when_b
 }
 
 #[test]
-fn guardian_subagent_approvals_append_guardian_specific_guidance() {
+fn auto_review_approvals_append_auto_review_specific_guidance() {
     let text = approval_text(
         AskForApproval::OnRequest,
-        ApprovalsReviewer::GuardianSubagent,
+        ApprovalsReviewer::AutoReview,
         &Policy::empty(),
         /*exec_permission_approvals_enabled*/ false,
         /*request_permissions_tool_enabled*/ false,
@@ -212,10 +212,10 @@ fn guardian_subagent_approvals_append_guardian_specific_guidance() {
 }
 
 #[test]
-fn guardian_subagent_approvals_omit_guardian_specific_guidance_when_approval_is_never() {
+fn auto_review_approvals_omit_auto_review_specific_guidance_when_approval_is_never() {
     let text = approval_text(
         AskForApproval::Never,
-        ApprovalsReviewer::GuardianSubagent,
+        ApprovalsReviewer::AutoReview,
         &Policy::empty(),
         /*exec_permission_approvals_enabled*/ false,
         /*request_permissions_tool_enabled*/ false,