Rebrand approvals reviewer config to auto-review (#18504)

### Why

Auto-review is the user-facing name for the approvals reviewer, but the
config/API value still exposed the old `guardian_subagent` name. That
made new configs and generated schemas point users at Guardian
terminology even though the intended product surface is Auto-review.

This PR updates the external `approvals_reviewer` value while preserving
compatibility for existing configs and clients.

### What changed

- Makes `auto_review` the canonical serialized value for
`approvals_reviewer`.
- Keeps `guardian_subagent` accepted as a legacy alias.
- Keeps `user` accepted and serialized as `user`.
- Updates generated config and app-server schemas so
`approvals_reviewer` includes:
  - `user`
  - `auto_review`
  - `guardian_subagent`
- Updates app-server README docs for the reviewer value.
- Updates analytics and config requirements tests for the canonical
auto_review value.


### Compatibility

Existing configs and API payloads using:

```toml
approvals_reviewer = "guardian_subagent"
```

continue to load and map to the Auto-review reviewer behavior. 

New serialization emits: 
```toml
approvals_reviewer = "auto_review" 
```

This PR intentionally does not rename the [features].guardian_approval
key or broad internal Guardian symbols. Those are split out for a
follow-up PR to keep this migration small and avoid touching large
TUI/internal surfaces.

**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

codex-rs/app-server/README.md

@@ -529,7 +529,7 @@ You can optionally specify config overrides on the new turn. If specified, these
 `approvalsReviewer` accepts:
 
 - `"user"` — default. Review approval requests directly in the client.
-- `"guardian_subagent"` — route approval requests to a carefully prompted subagent, which gathers relevant context and applies a risk-based decision framework before approving or denying the request.
+- `"auto_review"` — route approval requests to a carefully prompted subagent, which gathers relevant context and applies a risk-based decision framework before approving or denying the request. The legacy value `"guardian_subagent"` is still accepted for compatibility.
 
 ```json
 { "method": "turn/start", "id": 30, "params": {