fix: use valid model name for Vertex AI connectivity check (#919)

The `_test_vertex_connection` function used `gemini-3-flash-preview`
which does not exist, causing the countTokens API to return 404 and
the connectivity check to always fail. Replace with `gemini-2.5-flash`.

Author: eliasecchig

agent_starter_pack/cli/utils/gcp.py

@@ -120,7 +120,7 @@ def _test_vertex_connection(
 
     try:
         response = requests.post(
-            f"https://us-central1-aiplatform.googleapis.com/v1beta1/projects/{project}/locations/global/publishers/google/models/gemini-3-flash-preview:countTokens",
+            f"https://us-central1-aiplatform.googleapis.com/v1beta1/projects/{project}/locations/global/publishers/google/models/gemini-2.5-flash:countTokens",
             headers={
                 "Authorization": f"Bearer {token}",
                 "Content-Type": "application/json",