approvalsReviewer docs/schema prefer auto_review but Rust serializes AutoReview as guardian_subagent

What happened?

The app-server docs and generated schema describe approvalsReviewer as accepting "auto_review", with "guardian_subagent" described as the legacy compatibility value.

However, the Rust enum currently serializes ApprovalsReviewer::AutoReview as "guardian_subagent" and only accepts "auto_review" as a serde alias. The protocol test also asserts serialization to "guardian_subagent".

That means server-produced values can still use the legacy spelling even though the docs/schema present "auto_review" as canonical.

Expected behavior

The serialization behavior, tests, docs, and generated schema should agree on the canonical emitted value for ApprovalsReviewer::AutoReview. If "auto_review" is intended to be canonical, Rust serialization should emit it while continuing to accept "guardian_subagent" as a legacy alias.

Additional context

I searched open issues and PRs for terms including approvalsReviewer, auto_review, guardian_subagent, serialization, schema, and docs, and did not find an existing open item that appears to cover this mismatch.