fix: support us region routing

PiperOrigin-RevId: 890026941

Author: google-genai-bot

client.go

@@ -293,6 +293,8 @@ func NewInternalAPIClient(ctx context.Context, cc *ClientConfig) (*InternalAPICl
 		if cc.HTTPOptions.BaseURL == "" {
 			if cc.Location == "global" || cc.APIKey != "" {
 				cc.HTTPOptions.BaseURL = "https://aiplatform.googleapis.com/"
+			} else if cc.Location == "us" {
+				cc.HTTPOptions.BaseURL = fmt.Sprintf("https://aiplatform.%s.rep.googleapis.com/", cc.Location)
 			} else {
 				cc.HTTPOptions.BaseURL = fmt.Sprintf("https://%s-aiplatform.googleapis.com/", cc.Location)
 			}

client_test.go

@@ -963,6 +963,32 @@ func TestClientConfigHTTPOptions(t *testing.T) {
 			expectedBaseURL:    "https://aiplatform.googleapis.com/",
 			expectedAPIVersion: "v1beta1",
 		},
+		{
+			name: "Vertex AI Backend with us location",
+			clientConfig: ClientConfig{
+				Backend:     BackendVertexAI,
+				Project:     "test-project",
+				Location:    "us",
+				HTTPOptions: HTTPOptions{},
+				Credentials: &auth.Credentials{},
+			},
+			expectedBaseURL:    "https://aiplatform.us.rep.googleapis.com/",
+			expectedAPIVersion: "v1beta1",
+		},
+		{
+			name: "Vertex AI Backend with us location and base URL override",
+			clientConfig: ClientConfig{
+				Backend:  BackendVertexAI,
+				Project:  "test-project",
+				Location: "us",
+				HTTPOptions: HTTPOptions{
+					BaseURL: "https://my-custom-url.com/",
+				},
+				Credentials: &auth.Credentials{},
+			},
+			expectedBaseURL:    "https://my-custom-url.com/",
+			expectedAPIVersion: "v1beta1",
+		},
 		{
 			name: "Google AI Backend with HTTP Client Timeout and no HTTPOptions",
 			clientConfig: ClientConfig{