Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

This PR fixes an issue where custom object formats failed during serialization when nested inside a Union type (check #779).

Previously, the fjs_type keyword was used to support JavaScript Date objects, since Ajv does not natively validate them. This PR extends that logic to all objects implementing a .toJSON() method, ensuring they are serialized correctly (for example, MongoDB ObjectId).

Key Points

✅ Extends fjs_type handling beyond Date to any .toJSON()-capable object.
✅ Ensures correct serialization for custom formats like ObjectId.
✅ Backward compatibility is preserved since Date objects already implement .toJSON() (returning an ISO string).