fix: use aiplatform.user role for agent identity IAM permissions (#727)

Update agent identity setup to grant roles/aiplatform.user instead of
roles/aiplatform.expressUser. The user role provides broader access to
Vertex AI services including model prediction capabilities, while
expressUser is limited to simplified Express Mode features with API keys.

Author: eliasecchig

agent_starter_pack/deployment_targets/agent_engine/python/{{cookiecutter.agent_directory}}/app_utils/deploy.py

@@ -148,7 +148,7 @@ def setup_agent_identity(client: Any, project: str, display_name: str) -> Any:
     )
 
     roles = [
-        "roles/aiplatform.expressUser",
+        "roles/aiplatform.user",
         "roles/serviceusage.serviceUsageConsumer",
         "roles/browser",
         "roles/cloudapiregistry.viewer",