docs: update testing requirements status with existing tests

EXHAUSTIVE_TODO_LIST.md

@@ -4333,15 +4333,28 @@ After each atomic action:
 
 ### Epic 3: Error Propagation
 - [ ] Unit tests for ErrorDisplay component
-- [ ] Unit tests for error categorization
-- [ ] Integration tests for error boundaries
-- [ ] E2E tests for error recovery flows
+  - ⚠️ Component exists but no dedicated unit tests found
+  - ✅ Component is tested indirectly through error-boundary and error-handling E2E tests
+- [x] Unit tests for error categorization
+  - ✅ `apps/web/src/utils/apiErrorHandler.test.ts` exists with comprehensive tests for `parseApiError` and `getErrorCategory`
+- [x] Integration tests for error boundaries
+  - ✅ `apps/web/src/components/ErrorBoundary.test.tsx` exists
+  - ✅ `apps/web/src/features/playlists/components/PlaylistErrorBoundary.test.tsx` exists
+- [x] E2E tests for error recovery flows
+  - ✅ `apps/web/e2e/error-boundary.spec.ts` exists
+  - ✅ `apps/web/e2e/error-handling.spec.ts` exists
 
 ### Epic 4: State Ownership
 - [ ] Unit tests for useUser hook
+  - ⚠️ Hook exists (`apps/web/src/features/auth/hooks/useUser.ts`) but no dedicated unit tests found
+  - ✅ Related hook `useUsernameAvailability.test.ts` exists
+  - ✅ `useAuth.test.ts` exists which may test similar functionality
 - [ ] Integration tests for React Query migration
+  - ⚠️ React Query is integrated but comprehensive integration tests may be needed
 - [ ] E2E tests for state sync across tabs
+  - ⚠️ Multi-tab state sync tests may be needed
 - [ ] Performance tests for state updates
+  - ⚠️ Performance tests for state management may be needed
 
 ### Epic 5: Security & Robustness
 - [ ] Security audit for token storage changes