withMessageFormatter method
Maps errors to user-facing messages using formatter.
This is a convenience wrapper around mapError for structured formatter classes.
final validator = authRule.withMessageFormatter(const AuthErrorFormatter());
final result = validator.validate('');
// result.errorOrNull == 'Email is required'
Implementation
Formix<T, String> withMessageFormatter(ErrorMessageFormatter<E> formatter) =>
mapError(formatter.format);