test: add test for setLastVisitedChallenge

mobile-app/test/unit/learn_service_test.dart

@@ -0,0 +1,43 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:freecodecamp/models/learn/curriculum_model.dart';
+import 'package:freecodecamp/service/learn/learn_service.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  test('setLastVisitedChallenge saves correct data to SharedPreferences',
+      () async {
+    SharedPreferences.setMockInitialValues({});
+
+    final prefs = await SharedPreferences.getInstance();
+
+    final LearnService service = LearnService();
+
+    final block = Block(
+      name: 'Learn Greetings in Your First Day at the Office',
+      dashedName: 'learn-greetings-in-your-first-day-at-the-office',
+      description: ['block description'],
+      isStepBased: true,
+      challenges: [],
+      challengeTiles: [],
+      superBlock: SuperBlock(
+        dashedName: 'a2-english-for-developers',
+        name: 'A2 English for Developers',
+      ),
+    );
+
+    service.setLastVisitedChallenge(
+        'https://www.freecodecamp.dev/curriculum-data/v2/challenges/a2-english-for-developers/learn-greetings-in-your-first-day-at-the-office/6543aa3df5f028dba112f275.json',
+        block);
+
+    final result = prefs.getStringList('lastVisitedChallenge');
+
+    expect(result, [
+      'https://www.freecodecamp.dev/curriculum-data/v2/challenges/a2-english-for-developers/learn-greetings-in-your-first-day-at-the-office/6543aa3df5f028dba112f275.json',
+      'a2-english-for-developers',
+      'A2 English for Developers',
+      'learn-greetings-in-your-first-day-at-the-office',
+    ]);
+  });
+}