[codex] Improve preparation validation feedback

[jjoonleo]

Summary

• Show preparation edit validation messages under each affected tile
• Validate touched preparation names on blur and touched preparation times when the time chip is opened
• Keep the existing disabled Next button behavior while making invalid touched fields understandable
• Include visible draft preparation rows in form validation and serialization

Validation

• flutter analyze
• flutter test test/presentation/schedule_create/bloc/schedule_form_bloc_test.dart test/presentation/my_page/preparation_spare_time_edit/preparation_spare_time_edit_screen_test.dart test/presentation/schedule_create/schedule_spare_and_preparing_time/preparation_form/bloc/preparation_form_bloc_test.dart

Fixes #421