fix: mock MuxPlayer in tests to prevent jsdom Web Component errors (#10858)

The @mux/mux-player Web Component crashes jsdom with
`r.addEventListener is not a function` when mounted during tests,
causing 12 unhandled errors and CI failure despite all tests passing.
Mocking the component prevents it from mounting in the test environment.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: mgoudy91

apps/mux/frontend/src/index.test.tsx

@@ -19,6 +19,11 @@ vi.mock('contentful-management', () => ({
   })),
 }));
 
+// Mock MuxPlayer to avoid Web Component errors in jsdom
+vi.mock('@mux/mux-player-react', () => ({
+  default: vi.fn(() => null),
+}));
+
 // Mock the API client
 vi.mock('./util/apiClient', () => ({
   default: vi.fn().mockImplementation(() => ({