code: def test_backend_fallback_to_env_var(self, mock_get_connection):
      mock_get_connection.return_value = None
  
      backends = ensure_secrets_loaded()
      backend_classes = [
          backend.__class__.__name__ for backend in backends
      ]
      assert "SystemsManagerParameterStoreBackend" in backend_classes
  
      conn = Connection.get_connection_from_secrets(
          conn_id="test_mysql"
      )
  
      # Assert that SystemsManagerParameterStoreBackend.get_conn_uri was called
      mock_get_connection.assert_called_once_with(conn_id="test_mysql")
  
      assert (
          "mysql://airflow:airflow@host:5432/airflow" == conn.get_uri()
      )
  
imports: from __future__ import annotations
  from unittest import mock
  import pytest
  from airflow.configuration import ensure_secrets_loaded, initialize_secrets_backends
  from airflow.models import Connection, Variable
  from airflow.secrets.cache import SecretCache
  from tests.test_utils.config import conf_vars
  from tests.test_utils.db import clear_db_variables
label_desc: Use image node_id 6 for calling the TestConnectionsFromSecrets obj's underlying member method code with example usage: obj.test_backend_fallback_to_env_var(mock_get_connection) without return types