Fixes#17253
When clicking 'No {Field}' in a polymorphic relation picker, the
relation was not being cleared. This commit implements the missing
detach logic:
- RecordDetailMorphRelationSectionDropdownManyToOne: Call onSubmit with
newValue: null when no item is selected
- MorphRelationManyToOneFieldInput: Call persistMorphManyToOne with
valueToPersist: null for detach case
- useMorphPersistManyToOne: Implement actual detach logic that sets all
morph relation ID fields to null via updateOneRecord
Also adds unit tests for the useMorphPersistManyToOne hook.
bitloi
a4f5197e45