Ensure ObjectReferences are processed after their parent assets in Scene

[xyao-nv]

Summary

The bug was discovered in DLI course that ObjectReference cannot be added before parent Object when concat them into Scene construction. This MR adds ordering guarantees and a validation check to prevent this bug.

Detailed description

• Previously, the order in which assets were passed to Scene.add_assets() determined processing order, which could silently lead to incorrect behavior if an ObjectReference was listed before its parent.
• Now it sorts assets so that ObjectReference instances are always processed last, regardless of input order.
• Add an assertion that every ObjectReference in the list has its parent_asset included in the same add_assets() call. Raises a clear error message if not.
• Add tests.