Make Guardian risk score persistence opt-in (#40911)

## What changed

- Add the `features.guardianv2.persist_scores` configuration option for debugging.
- Default the option to `false`, so Guardian V2 reviewed actions and risk scores are only written to rollout files when explicitly enabled and the session is not ephemeral.

## Testing

- Cover configuration deserialization, default non-persistence, and opt-in persistence for nested code-mode actions.

GitOrigin-RevId: 452af99ef14553d45140340ececc2913389124d0