The validation_regex field was not being enforced for URL type custom
fields. This fix adds regex validation in two places:
1. to_form_field() - Applies regex validator to form fields (UI validation)
2. validate() - Applies regex check in model validation (API/programmatic)
Note: The original issue reported UI validation only, but this fix also
adds API validation for consistency with text field behavior and to
ensure data integrity across all entry points.