chore: For bug bashing with staging express mode API

Co-authored-by: Kathy Wu <wukathy@google.com>
PiperOrigin-RevId: 895510871

Author: wukath

src/google/adk/cli/utils/gcp_utils.py

@@ -30,7 +30,9 @@
 from google.cloud import resourcemanager_v3
 import requests
 
-_VERTEX_AI_ENDPOINT = "https://{location}-aiplatform.googleapis.com/v1beta1"
+_VERTEX_AI_ENDPOINT = (
+    "https://staging-{location}-aiplatform.sandbox.googleapis.com/v1beta1"
+)
 
 
 def check_adc() -> bool: