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/protocol/src/config_types.rs

@@ -89,7 +89,7 @@ pub enum ApprovalsReviewer {
     User,
     #[serde(rename = "auto_review", alias = "guardian_subagent")]
     #[strum(serialize = "auto_review")]
-    GuardianSubagent,
+    AutoReview,
 }
 
 impl JsonSchema for ApprovalsReviewer {
@@ -608,8 +608,7 @@ mod tests {
             "\"user\""
         );
         assert_eq!(
-            serde_json::to_string(&ApprovalsReviewer::GuardianSubagent)
-                .expect("serialize reviewer"),
+            serde_json::to_string(&ApprovalsReviewer::AutoReview).expect("serialize reviewer"),
             "\"auto_review\""
         );
 
@@ -620,7 +619,7 @@ mod tests {
             let expected = if value == "user" {
                 ApprovalsReviewer::User
             } else {
-                ApprovalsReviewer::GuardianSubagent
+                ApprovalsReviewer::AutoReview
             };
             assert_eq!(expected, reviewer);
         }