Fix test error

[lxiao-coder]

I am not sure if it is an issue happen to me only. But before the change, the test has error.
We need to mock session.enter as it is a content manager.

backend/app/backend_pre_start.py

with Session(db_engine) as session:
            # Try to create session to check if DB is awake
            session.exec(select(1))