Do mocks not work inside apps?

[mccalluc]

Working on

• Playwright test for --demo #105

and having some trouble: I could be misusing mocks, but I'm wondering if the problem is that the app isn't code we run directly from the pytest process: Instead it's referenced from create_app_fixture.

To close:

• Make a minimal reproducer
• Add a note in lessons-learned: Document what we've learned #118