fix: support us region routing

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/google-gemini/private-dotnet-genai/pull/56 from google-gemini:integration 95fde92a3f597a59389fc8259d69398a685b7b91
PiperOrigin-RevId: 885956883

Author: google-genai-bot

Google.GenAI.Tests/HttpApiClientTest.cs

@@ -270,6 +270,29 @@ public void VertexConstructor_LocationGlobal_SetsCorrectBaseUrl() {
       Assert.AreEqual("https://aiplatform.googleapis.com", client.HttpOptions.BaseUrl);
     }
 
+    [TestMethod]
+    public void VertexConstructor_LocationUs_SetsCorrectBaseUrl() {
+      var mockCredential = new Mock<ICredential>();
+      var client = new HttpApiClient(vertexAI: true, project: "my-project", location: "us", credentials: mockCredential.Object);
+
+      Assert.AreEqual("my-project", client.Project);
+      Assert.AreEqual("us", client.Location);
+      Assert.IsTrue(client.VertexAI);
+      Assert.AreEqual("https://aiplatform.us.rep.googleapis.com", client.HttpOptions.BaseUrl);
+    }
+
+    [TestMethod]
+    public void VertexConstructor_LocationUsAndCustomHttpOptions_SetsCorrectBaseUrl() {
+      var mockCredential = new Mock<ICredential>();
+      var customOptions = new Types.HttpOptions { BaseUrl = "https://my-custom-url.com/" };
+      var client = new HttpApiClient(vertexAI: true, project: "my-project", location: "us", credentials: mockCredential.Object, httpOptions: customOptions);
+
+      Assert.AreEqual("my-project", client.Project);
+      Assert.AreEqual("us", client.Location);
+      Assert.IsTrue(client.VertexAI);
+      Assert.AreEqual("https://my-custom-url.com/", client.HttpOptions.BaseUrl);
+    }
+
     [TestMethod]
     public void Constructor_HttpOptions_BaseUrlResourceScopeWithoutBaseUrl_ThrowsArgumentException() {
       var customOptions = new Types.HttpOptions { BaseUrlResourceScope = Types.ResourceScope.Collection, BaseUrl = null };

Google.GenAI/ApiClient.cs

@@ -316,6 +316,10 @@ internal static HttpOptions GetDefaultHttpOptions(bool vertexAI, string? locatio
         {
           defaultHttpOptions.BaseUrl = "https://aiplatform.googleapis.com";
         }
+        else if (location!.Equals("us"))
+        {
+          defaultHttpOptions.BaseUrl = $"https://aiplatform.{location}.rep.googleapis.com";
+        }
         else
         {
           defaultHttpOptions.BaseUrl = $"https://{location}-aiplatform.googleapis.com";