docs(http): add http.client Testing section with Flutter testing cookbook recommendation

pkgs/http/CHANGELOG.md

@@ -4,6 +4,7 @@
   parameter is only added for text and XML media types. This brings the
   behavior of `package:http` in line with RFC-8259.
 * Export `MediaType` from `package:http_parser`.
+* Added a section on testing to `README.md`.
 
 ## 1.5.0
 

pkgs/http/README.md

@@ -82,6 +82,56 @@ class UserAgentClient extends http.BaseClient {
   }
 }
 ```
+## Testing
+
+For better testability, especially in Flutter applications, it's recommended
+to use a [Client][] instance rather than the top-level functions like
+`http.get()` or `http.post()`. This approach makes it easier to mock HTTP
+requests in your tests.
+
+```dart
+// Instead of using static methods (harder to test):
+// var response = await http.get(Uri.https('example.com', 'data.json'));
+
+// Use a Client instance (easier to test):
+class DataService {
+  final http.Client client;
+
+  DataService({http.Client? client}) : client = client ?? http.Client();
+
+  Future<String> fetchData() async {
+    var response = await client.get(Uri.https('example.com', 'data.json'));
+    return response.body;
+  }
+}
+```
+
+In your tests, you can easily mock the HTTP client:
+
+```dart
+import 'package:http/testing.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('fetchData returns expected data', () async {
+    final mockClient = MockClient((request) async {
+      return http.Response('{"key": "value"}', 200);
+    });
+
+    final dataService = DataService(client: mockClient);
+    final result = await dataService.fetchData();
+
+    expect(result, '{"key": "value"}');
+  });
+}
+```
+
+> [!TIP]
+> You can also use `package:mockito` to mock `http.Client`.
+> For more detailed guidance, see the
+> [Flutter testing cookbook](https://docs.flutter.dev/cookbook/testing/unit/mocking),
+> which demonstrates best practices for mocking HTTP requests.
+
 
 ## Retrying requests