test(tests): add comprehensive unit tests for various models and repositories, enhancing test coverage and ensuring correct functionality

test/core/comments/models/comment_update_model_test.dart

@@ -52,5 +52,45 @@ void main() {
 
       expect(model1 == model2, false);
     });
+
+    test('fromJson - создает модель из JSON', () {
+      final json = {'body': 'Test comment body'};
+      final model = CommentUpdateModel.fromJson(json);
+
+      expect(model.body, 'Test comment body');
+    });
+
+    test('fromJson - обрабатывает пустую строку', () {
+      final json = {'body': ''};
+      final model = CommentUpdateModel.fromJson(json);
+
+      expect(model.body, '');
+    });
+
+    test('fromJson - обрабатывает длинный текст', () {
+      final longBody = 'A' * 1000;
+      final json = {'body': longBody};
+      final model = CommentUpdateModel.fromJson(json);
+
+      expect(model.body, longBody);
+      expect(model.body.length, 1000);
+    });
+
+    test('toJson - преобразует модель в JSON', () {
+      final model = CommentUpdateModel(body: 'Test comment body');
+      final json = model.toJson();
+
+      expect(json, isA<Map<String, dynamic>>());
+      expect(json['body'], 'Test comment body');
+    });
+
+    test('toJson и fromJson - обратное преобразование', () {
+      final originalModel = CommentUpdateModel(body: 'Test comment');
+      final json = originalModel.toJson();
+      final restoredModel = CommentUpdateModel.fromJson(json);
+
+      expect(restoredModel.body, originalModel.body);
+      expect(restoredModel, originalModel);
+    });
   });
 }