Create mocks for the entire checkout extension API

Author: kumar303

examples/testing/checkout-basic-testing-example/extensions/checkout-basic-testing-example/tests/Checkout.test.ts

@@ -1,6 +1,9 @@
 import {expect, test, vi, beforeEach, afterEach} from 'vitest';
 import {setUpExtensionForTests} from '@shopify/ui-extensions-tester';
-import {createAppMetafields} from '@shopify/ui-extensions-tester/checkout';
+import {
+  createAppMetafields,
+  createApplyMetafieldChangeResult,
+} from '@shopify/ui-extensions-tester/checkout';
 
 const extension = setUpExtensionForTests('purchase.checkout.block.render');
 
@@ -34,7 +37,9 @@ test('shows the existing birth date from appMetafields', async () => {
 });
 
 test('calls applyMetafieldChange with the entered date when clicking Enter', async () => {
-  const applyMetafieldChange = vi.fn().mockResolvedValue({type: 'success'});
+  const applyMetafieldChange = vi
+    .fn()
+    .mockResolvedValue(createApplyMetafieldChangeResult());
   extension.mockGlobalShopify({applyMetafieldChange});
 
   await extension.render();