test(data-fabric): make ApiClient mocks constructable for vitest 4 (#571)

The DataFabric directory and roles service tests landed after the vitest-4
mock migration, so they still mocked ApiClient with an arrow function:
`vi.mocked(ApiClient).mockImplementation(() => mockApiClient as unknown as ApiClient)`.
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 both suites failed to run.

Converted both to constructable regular functions, matching every other
service test. Full unit suite (2021 tests) passes on vitest 4.1.9.

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

Author: Sarath1018

tests/unit/services/data-fabric/directory.test.ts

@@ -22,7 +22,7 @@ describe('DataFabricDirectoryService 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; });
     directoryService = new DataFabricDirectoryService(instance);
   });
 

tests/unit/services/data-fabric/roles.test.ts

@@ -18,7 +18,7 @@ describe('DataFabricRoleService 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; });
     rolesService = new DataFabricRoleService(instance);
   });