fix(api): add clearAllMocks and explicit return type to GroupsModule

- Add jest.clearAllMocks() in beforeEach of both spec files to prevent
  call-history state from leaking between tests as the suites grow
- Define GroupSummary interface and use it as explicit return type for
  GroupsService.list() to satisfy TypeScript strict typing requirements

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/api/src/modules/groups/groups.controller.spec.ts

@@ -10,6 +10,10 @@ const mockService = { list: jest.fn().mockResolvedValue(mockGroups) };
 describe('GroupsController', () => {
   let controller: GroupsController;
 
+  beforeEach(() => {
+    jest.clearAllMocks();
+  });
+
   beforeEach(async () => {
     const module: TestingModule = await Test.createTestingModule({
       controllers: [GroupsController],