feat: Migrate vertex_ai_session_service to use Agent Engine SDK

PiperOrigin-RevId: 814948921

Author: DeanChensj

src/google/adk/cli/fast_api.py

@@ -105,8 +105,8 @@ def _parse_agent_engine_resource_name(agent_engine_id_or_resource_name):
  agent_engine_id = agent_engine_id_or_resource_name.split("/")[-1]
  else:
  envs.load_dotenv_for_agent("", agents_dir)
- project = os.environ["GOOGLE_CLOUD_PROJECT"]
- location = os.environ["GOOGLE_CLOUD_LOCATION"]
+ project = os.environ.get("GOOGLE_CLOUD_PROJECT", None)
+ location = os.environ.get("GOOGLE_CLOUD_LOCATION", None)
  agent_engine_id = agent_engine_id_or_resource_name
  return project, location, agent_engine_id