Adds comprehensive unit tests for the internal utility function shouldRenderFormState.
This ensures correctness of form state reactivity based on subscription flags and root-level validation mode.

• Adds test cases for:
  • Empty formState returns true
  • Changed keys exceed subscription length
  • Matching subscribed keys (including isRoot logic)
• Covers all logical branches (including default return, VALIDATION_MODE.all)
• Groups root subscriber–specific scenarios under a dedicated describe block for clarity

This logic determines whether a form should re-render based on field-level or global state subscription.
Having full test coverage prevents subtle bugs and provides confidence in future refactors.