test(frontend): add FormlyRepeatDndComponent unit tests

[Roshan3580]

Summary

Adds unit tests for FormlyRepeatDndComponent.

Closes #5464

Changes

• Added repeat-dnd.component.spec.ts
• Added a test to verify the component is created successfully
• Added tests covering the early-return behavior when:
  • previousIndex === currentIndex
  • model is undefined
• Added a test covering the happy path to verify:
  • model items are reordered correctly
  • field.fieldGroup items are reordered correctly
  • FormArray controls are reordered correctly
  • props.reorder() is called once

Testing

Executed:

yarn test --include='**/repeat-dnd.component.spec.ts'
yarn lint

[Yicong-Huang]

@Roshan3580 please use our PR template. Please feel free to reopen once it is ready. Thanks!