feat: Migrate vertex ai session service to use agent engine sdk

PiperOrigin-RevId: 816259798

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