When default value is null but source has a value (e.g., description field), the merge was discarding the source value because typeof null !== typeof string. Now properly accepts source values for nullable fields.
- Add getFirstValidationError utility for nested FieldErrors extraction - Add mergeWithDefaults utilities (deepMergeWithDefaults, type guards) - Add useValidationErrorHandler hook for toast + tab navigation - Add FormSelect component with Controller integration - Add FormTextarea component with register integration - Refactor AgentTypeForm to use new utilities - Remove verbose debug logging (now handled by hook) - Add comprehensive tests (53 new tests, 100 total)
