How to distinguish marimo notebooks from any other python files robustly and efficiently?

The marimo edit server uses the following heuristic:

https://github.com/marimo-team/marimo/blob/9668948b34ca156a3e8cef57507f3122fb9dbdfa/marimo/_server/files/os_file_system.py#L94-L99

I think this can't be a robust algorithm to test whether a python file is a marimo notebook.
For example, run _is_marimo_file on this file exactly will return True 🤔

There are more checks that could be done. It’s not perfect, but I think the downstream use case of it, it doesn’t need to be perfect

We thought about saving marimo notebooks with a custom extension for this reason (.mo.py), but such files can't be imported by Python's import statement.