fix(core): prevent cleared fields from re-defaulting when another field is cleared (#5125 regression)

[yap-house]

Reasons for making this change

This fixes a regression introduced in #5136.
After clearing field A, clearing field B caused field A to be re-populated with its schema default.
The fix uses _set(formData, path, undefined) instead of _unset to keep the cleared key present in state, preventing mergeDefaultsWithFormData from re-applying the default. A new private helper omitUndefinedLeaves() strips undefined-valued keys before AJV validation to preserve the #4518 fix.

fixes #5125

Checklist

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