The nest assign_object API now removes existing spatial containment and
aggregate relationships before creating the nest, matching the behavior
documented in its docstring and consistent with aggregate.assign_object.
Fix#7248
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
order of objects in .RelatedObjects is important (e.g. for cost items, it's the order of their appearance), so we should maintain it and cannot use sets for .RelatedObjects