fix: #3552007 Entity reference field diff missing when referenced entity cannot be loaded

EntityReferenceFieldBuilder skipped rendering entirely when the referenced entity object could not be loaded (e.g. it had been deleted), even though the target_id was still present, hiding real changes from the revision diff. Fall back to showing the raw target ID in that case, and add test coverage for a deleted reference target.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com

Closes #3552007