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/app-server/src/config_api.rs

@@ -459,7 +459,7 @@ mod tests {
             ]),
             allowed_approvals_reviewers: Some(vec![
                 CoreApprovalsReviewer::User,
-                CoreApprovalsReviewer::GuardianSubagent,
+                CoreApprovalsReviewer::AutoReview,
             ]),
             allowed_sandbox_modes: Some(vec![
                 CoreSandboxModeRequirement::ReadOnly,
@@ -524,7 +524,7 @@ mod tests {
             mapped.allowed_approvals_reviewers,
             Some(vec![
                 codex_app_server_protocol::ApprovalsReviewer::User,
-                codex_app_server_protocol::ApprovalsReviewer::GuardianSubagent,
+                codex_app_server_protocol::ApprovalsReviewer::AutoReview,
             ])
         );
         assert_eq!(