## What changed
- Add model message metadata for built-in tool descriptions.
- Use the active step model's catalog description for `send_user_message_async`, including after a mid-turn model change.
- Fall back to the built-in description when catalog metadata is absent, while preserving an explicitly empty description.
## Testing
- Cover catalog serialization, fallback and empty-value behavior, and model changes within a turn.
GitOrigin-RevId: 72b953214ab9708931065321debf12133f8d2d40