Align mock type with actual usage (sync | async) for next/headers.headers

Author: heatlikeheatwave

packages/nextjs/src/__tests__/keyless-custom-headers.test.ts

@@ -45,7 +45,8 @@ vi.mock('next/headers', () => ({
   })),
 }));
 
-const mockHeaders = headers as unknown as MockedFunction<() => Promise<MockHeaders>>;
+type MockHeadersFn = () => MockHeaders | Promise<MockHeaders>;
+const mockHeaders = headers as unknown as MockedFunction<MockHeadersFn>;
 
 // Type for mocking Next.js headers
 interface MockHeaders {