Route Guardian inference through dedicated endpoints (#40892)

## What changed

- Add the opt-in `features.guardianv2.free_guardian` setting to route eligible
  Guardian reviews to `/guardian` and asynchronous classifications to
  `/guardian-classifier`.
- Support endpoint selection for both HTTP and WebSocket Responses clients,
  including endpoint-aware connection reuse.
- Keep `/responses` for API-key authentication, custom provider URLs,
  non-Guardian models, and configurations where the option is disabled.
- Omit routing hints and `service_tier` from requests sent to the dedicated
  Guardian endpoints.

## Testing

- Cover route selection across authentication modes, provider URLs, HTTP
  fallback, WebSocket review sessions, and classifier sampling.

GitOrigin-RevId: 9c88673c710d009e0239559b9245a4e6c4f4befc