fix: prevent field array ghost elements with keepDirtyValues

[a28689604]

Proposed Changes

As mentioned in issue #12489, when using useFieldArray with the values prop and resetOptions: { keepDirtyValues: true }, removed field array items were being re-inserted as "ghost" elements on subsequent appends.

Root cause

During reset with keepDirtyValues: true, the logic iterates through _names.mount (which contains field names like test.0.value, test.1.value) to determine which values to preserve. However:

1. When array items are removed, their field names remain in _names.mount
2. The dirtyFields state is also preserved from before the reset
3. During the keepDirtyValues loop, the code attempted to update fields that no longer existed in _formValues
4. For clean fields, calling setValue() with a path to a non-existent array index created ghost elements (e.g., test.1.value created [{ value: undefined }])

Fixes

• Only preserve dirty values if the field still exists in current form values (existingValue !== undefined)
• Only update clean fields if they exist in the new reset values (newValue !== undefined)
• Skip processing entirely for fields that have been removed from the form
  This prevents the code from creating array elements for field paths that no longer exist.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Fixes #12489

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes

[Copilot]

Pull request overview

This PR fixes a bug where removed field array items were being re-inserted as "ghost" elements during form reset with keepDirtyValues enabled. The issue occurred because stale field names remained in _names.mount after removal, and the reset logic attempted to set values for non-existent array indices, inadvertently creating new array elements.

Key changes:

• Modified the keepDirtyValues logic to check if field values exist before setting them
• Added comprehensive test coverage for the field array ghost elements scenario

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/logic/createFormControl.ts	Added undefined checks to prevent setting values for non-existent field paths, fixing the ghost elements bug
src/tests/useFieldArray/remove.test.tsx	Added comprehensive test that reproduces and validates the fix for the ghost elements issue

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[bluebill1049]

can use the isUndefined util function