Add unit tests for Backend

Author: andrewheard

FirebaseAI/Sources/Types/Public/Backend.swift

@@ -48,5 +48,3 @@ public struct Backend {
     self.location = location
   }
 }
-
-extension Backend: Equatable {}

FirebaseAI/Tests/Unit/Types/BackendTests.swift

@@ -0,0 +1,56 @@
+// Copyright 2025 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 XCTest
+
+@testable import FirebaseAI
+
+final class BackendTests: XCTestCase {
+  func testVertexAI_defaultLocation() {
+    let expectedAPIConfig = APIConfig(
+      service: .vertexAI(endpoint: .firebaseVertexAIProd),
+      version: .v1beta
+    )
+
+    let backend = Backend.vertexAI()
+
+    XCTAssertEqual(backend.apiConfig, expectedAPIConfig)
+    XCTAssertEqual(backend.location, "us-central1")
+  }
+
+  func testVertexAI_customLocation() {
+    let expectedAPIConfig = APIConfig(
+      service: .vertexAI(endpoint: .firebaseVertexAIProd),
+      version: .v1beta
+    )
+    let customLocation = "europe-west1"
+
+    let backend = Backend.vertexAI(location: customLocation)
+
+    XCTAssertEqual(backend.apiConfig, expectedAPIConfig)
+    XCTAssertEqual(backend.location, customLocation)
+  }
+
+  func testGoogleAI() {
+    let expectedAPIConfig = APIConfig(
+      service: .developer(endpoint: .firebaseVertexAIProd),
+      version: .v1beta
+    )
+
+    let backend = Backend.googleAI()
+
+    XCTAssertEqual(backend.apiConfig, expectedAPIConfig)
+    XCTAssertNil(backend.location)
+  }
+}