test(notifications): make ApiClient mock constructable for vitest 4 (#570)

The Notifications service test landed after the vitest-4 mock migration, so
it still mocked ApiClient with an arrow function:
`vi.mocked(ApiClient).mockImplementation(() => mockApiClient ...)`. Vitest 4
constructs mock implementations via the function's [[Construct]] slot, which
arrow functions lack, so `new ApiClient(...)` in the service constructor threw
`() => ... is not a constructor` and the suite failed to run.

Converted to a constructable regular function, matching every other service
test. Full unit suite (1992 tests) passes on vitest 4.1.9.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Sarath1018

tests/unit/services/notification/notifications.test.ts

@@ -34,7 +34,7 @@ describe('NotificationService Unit Tests', () => {
   beforeEach(() => {
     const { instance } = createServiceTestDependencies();
     mockApiClient = createMockApiClient();
-    vi.mocked(ApiClient).mockImplementation(() => mockApiClient as unknown as ApiClient);
+    vi.mocked(ApiClient).mockImplementation(function () { return mockApiClient as unknown as ApiClient; });
     vi.mocked(PaginationHelpers.getAll).mockReset();
 
     notificationService = new NotificationService(instance);