flutter freezed multi version

freezed artifice

```dart
abstract class School {
  int version = 1;

  School();

    factory School.fromJson(Map<String, dynamic> json) {
    switch (json['version'] as int) {
      case 1:
        return SchoolEntity1.fromJson(json);
      case 2:
        return SchoolEntity2.fromJson(json);
      default:
        throw Exception('Unsupported version: ${json['version']}');
    }
  }
}

@unfreezed
class SchoolEntity1 extends School with _$SchoolEntity1 {
  SchoolEntity1._();
  factory SchoolEntity1({
    @Default(1) int version,
    required String name,
    required String teacher,
  }) = _SchoolEntity1;

  factory SchoolEntity1.fromJson(Map<String, dynamic> json) => _$SchoolEntity1FromJson(json);
}

@unfreezed
class SchoolEntity2 extends School with _$SchoolEntity2 {
  SchoolEntity2._();

  factory SchoolEntity2({
    @Default(2) int version,
    required String name,
    required String teacher,
    @Default("") String principal,
  }) = _SchoolEntity2;

  factory SchoolEntity2.fromJson(Map<String, dynamic> json) => _$SchoolEntity2FromJson(json);

  factory SchoolEntity2.fromSchoolEntity1(SchoolEntity1 entity1) {
    return SchoolEntity2(
      name: entity1.name,
      teacher: entity1.teacher,
    );
  }
}
```

here the test
```dart
void main() {
  group('School', () {
    test('should create School instance with correct values', () {
      Map<String, dynamic> sj = {
        'version': 1,
        'name': 'Test School',
        'teacher': 'Test Teacher',
      };

      SchoolEntity1 school1 = SchoolEntity1.fromJson(sj);
      expect(school1.version, equals(1));

      SchoolEntity2 school2;
      school2 = SchoolEntity2.fromSchoolEntity1(school1);
      expect(school2.version, equals(2));
    });
  });
}
```