## What changed
Expose the backend-provided `generationId` on image-generation items and preserve it through legacy completion events, thread history reconstruction, and rollout migration. Default missing IDs to `None` for older history.
Update the generated JSON schemas, TypeScript types, and Python models to include the nullable field.
## Testing
Extend coverage for ID serialization round trips, completed-item notifications, persisted and resumed history, and legacy rollout migration.
GitOrigin-RevId: a7f06faa7ac6833417b257a535d2901c03b79135