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/tui/src/chatwidget.rs

@@ -9042,7 +9042,7 @@ impl ChatWidget {
                             "Same workspace-write permissions as Default, but eligible `on-request` approvals are routed through the auto-reviewer subagent."
                                 .to_string(),
                         ),
-                        is_current: current_review_policy == ApprovalsReviewer::GuardianSubagent
+                        is_current: current_review_policy == ApprovalsReviewer::AutoReview
                             && Self::preset_matches_current(
                                 current_approval,
                                 current_sandbox,
@@ -9052,7 +9052,7 @@ impl ChatWidget {
                             preset.approval,
                             preset.sandbox.clone(),
                             "Auto-review".to_string(),
-                            ApprovalsReviewer::GuardianSubagent,
+                            ApprovalsReviewer::AutoReview,
                         ),
                         dismiss_on_select: true,
                         disabled_reason: approval_disabled_reason