test(react-overflow): fix useOverflowItem registration test

The cleanup now invokes the unregister function returned by registerItem, so
the mocked registerItem must return one too (the real one always does).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: bsunderhus

packages/react-components/react-overflow/library/src/useOverflowItem.test.tsx

@@ -17,7 +17,8 @@ const mockContextValue = (options: Partial<OverflowContextValue> = {}) =>
 
 describe('useOverflowItem', () => {
   it('should register item', () => {
-    const registerItemMock = jest.fn();
+    // registerItem returns an unregister cleanup, which the hook invokes on unmount.
+    const registerItemMock = jest.fn(() => jest.fn());
     const value = mockContextValue({ registerItem: registerItemMock });
     renderHook(
       () => {