add license header to test file

Author: russellwheatley

packages/ai/__tests__/backend.test.ts

@@ -1,3 +1,19 @@
+/**
+ * @license
+ * Copyright 2024 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
 import { describe, it, expect } from '@jest/globals';
 import { GoogleAIBackend, VertexAIBackend } from '../lib/backend';
 import { BackendType } from 'lib/public-types';