我找到了一个解决方案--去掉扩展中的方法。
实体：

class UserEntity {
  final int id;
  final String email;
  final String? photoUrl;

  const UserEntity({
    required this.id,
    required this.email,
    this.photoUrl,
  });
}

extension UserEntityCopy on UserEntity {
  UserEntity copyWith({
    int? id,
    String? email,
    String? photoUrl,
  }) {
    return UserEntity(
      id: id ?? this.id,
      email: email ?? this.email,
      photoUrl: photoUrl ?? this.photoUrl,
    );
  }
}

型号：

@Freezed()
class UserModel extends UserEntity with _$UserModel {
  const factory UserModel({
    @JsonKey(name: 'id', required: true, disallowNullValue: true) required int id,
    required String email,
    String? photoUrl,
  }) = _UserModel;

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