Fixes #4709: Boolean fields now remain undefined or get proper defaults when switching between schema variants with mergeDefaultsIntoFormData set to useDefaultIfFormDataUndefined

Reasons for making this change

This PR fixes a critical bug where boolean fields in discriminated unions (oneOf/anyOf) were incorrectly being set to empty objects {} instead of remaining undefined or receiving proper boolean default values when users switch between schema options.

Problem:

• When using mergeDefaultsIntoFormData: "useDefaultIfFormDataUndefined"
• Boolean fields that exist across multiple oneOf/anyOf schema variants
• Switching from one schema variant to another causes undefined boolean fields to be set to {}
• This breaks form validation and data integrity
• Only affects undefined/missing boolean fields (existing true/false values were preserved correctly)

Solution:
Updated the form data merging logic in @rjsf/utils to properly handle boolean field types when sanitizing data for new schemas, ensuring undefined boolean fields remain undefined or get appropriate default values instead of being converted to empty objects.

Testing:

• Added comprehensive test cases covering the oneOf boolean field scenarios
• Verified existing boolean values (true/false) continue to work correctly
• Ensured other field types are not affected by the changes

Fixes #4709

Checklist

• [ ] I'm updating documentation
  • [ ] I've checked the rendering of the Markdown text I've added
• I'm adding or updating code
  • I've added and/or updated tests. I've run npx nx run-many --target=build --exclude=@rjsf/docs && npm run test:update to update snapshots, if needed.
  • [ ] I've updated docs if needed
  • I've updated the changelog with a description of the PR
• [ ] I'm adding a new feature
  • [ ] I've updated the playground with an example use of the feature