Add Fantom integration test for FabricUIManager

Summary:
Converts the Jest tests for getFabricUIManager() to Fantom integration
tests using the real native Fabric runtime. Tests that rely on
jest.resetModules() to simulate an uninitialized environment are
omitted, as Fantom always provides a fully initialized runtime.

Changelog: [Internal]

Differential Revision: D94887591

Author: sammy-SC

packages/react-native/Libraries/ReactNative/__tests__/FabricUIManager-itest.js

@@ -0,0 +1,70 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict-local
+ * @format
+ */
+
+import '@react-native/fantom/src/setUpDefaultReactNativeEnvironment';
+
+import {getFabricUIManager} from '../FabricUIManager';
+
+// NOTE: The test that verifies getFabricUIManager() returns undefined when
+// global.nativeFabricUIManager is not set cannot be converted to a Fantom
+// integration test. That test relies on jest.resetModules() to clear the
+// module-level singleton (nativeFabricUIManagerProxy). In the Fantom
+// environment, global.nativeFabricUIManager is always set up by the runtime,
+// and there is no module reset mechanism available. That test exercises a code
+// path (getFabricUIManager returning undefined) that is not reachable in a
+// fully-initialized Fantom environment.
+
+describe('getFabricUIManager', () => {
+  it('should return an object when the global binding is set in the Fantom environment', () => {
+    // In the Fantom environment, global.nativeFabricUIManager is always set by
+    // the native runtime, so getFabricUIManager() must return a non-null object.
+    const fabricUIManager = getFabricUIManager();
+    expect(fabricUIManager).not.toBeNull();
+    expect(fabricUIManager).not.toBeUndefined();
+    expect(typeof fabricUIManager).toBe('object');
+  });
+
+  it('should return the same object on subsequent calls', () => {
+    // getFabricUIManager() caches the proxy object in a module-level variable.
+    // Subsequent calls should return the same reference.
+    const first = getFabricUIManager();
+    const second = getFabricUIManager();
+    expect(first).toBe(second);
+  });
+
+  it('should cache the createNode property so the same reference is returned each time', () => {
+    // The proxy created by getFabricUIManager() uses defineLazyObjectProperty
+    // to cache properties from the underlying global binding. Accessing the
+    // same property twice should return the same reference even if the
+    // underlying binding produces a new host function on each access.
+    const fabricUIManager = getFabricUIManager();
+    expect(fabricUIManager).not.toBeNull();
+    expect(fabricUIManager).not.toBeUndefined();
+
+    if (fabricUIManager != null) {
+      const firstCreateNode = fabricUIManager.createNode;
+      const secondCreateNode = fabricUIManager.createNode;
+      expect(firstCreateNode).toBe(secondCreateNode);
+    }
+  });
+
+  it('should expose the createNode method', () => {
+    // Verify that the returned proxy exposes createNode as a function.
+    // createNode is one of the CACHED_PROPERTIES in FabricUIManager.js and
+    // is provided by the Fantom native runtime.
+    const fabricUIManager = getFabricUIManager();
+    expect(fabricUIManager).not.toBeNull();
+    expect(fabricUIManager).not.toBeUndefined();
+
+    if (fabricUIManager != null) {
+      expect(typeof fabricUIManager.createNode).toBe('function');
+    }
+  });
+});