Browser suite fails when one worker runs a test outside browser/ between two browser tests #282

Goal

Make the browser suite pass when a pytest command names a file outside server/ui/tests/browser/ between two browser files. Today that command fails every later browser test's setup with SynchronousOnlyOperation. The implementer found that pytest 9.1.x binds the browser conftest's fixtures at collection (pytest-dev/pytest#14635, fixed in 9.2.0), so CI's -n auto -m browser ui cannot hit it and the run order does not matter; the PR carries the --collect-only evidence.

Current PHP behavior

• PHP has no counterpart. This is the Django port's browser test harness.
• During kiosk_screens.py reports overlapping elements and text cut short with an ellipsis #256 (master d2355a0), one command run from server/ gave 52 SynchronousOnlyOperation errors at the setup of test_kiosk_names.py's tests. The command is uv run pytest -m browser -p no:xdist ui/tests/browser/test_shell.py ui/tests/test_kiosk_screens_tool.py ui/tests/browser/test_kiosk_names.py. It was observed once and has not been reproduced since.
• The order that failed is a test under browser/, then a test outside it (ui/tests/test_kiosk_screens_tool.py), then browser/ again, all in one worker.
• The first suspect, refuted by the implementer, was that server/ui/tests/browser/conftest.py sets DJANGO_ALLOW_ASYNC_UNSAFE in its _async_unsafe fixture (around line 165), and that the setting is not in force for the later browser tests once a test outside that folder has run.

Scope

• Reproduce the failure with the command above on current master, and record the first error.
• Find the cause, and state the hypothesis that proved true in the PR.
• Fix it, with a regression check that goes red before the fix and green after it.

Out of scope

• Other browser flakes, such as the photo race in test_photos.py and test_kiosk_thumbnails.py.

Acceptance criteria

• The command above passes. Evidence is its output in the PR.
• A regression check fails on master and passes with the fix. Evidence is the check.
• The full browser suite passes on SQLite and on Postgres, since the browser conftest is shared. Evidence is both runs' summaries in the PR.

Risks and notes

• If the command passes on current master, look for an order that does fail, such as another test outside browser/ run between two browser tests, before calling it fixed, or close the ticket with the evidence.
• A fix that turns off Django's async safety check for the whole suite would hide real bugs in non-browser tests; scope it to the browser tests.

Dependencies

• Found by kiosk_screens.py reports overlapping elements and text cut short with an ellipsis #256.

References

• server/ui/tests/browser/conftest.py, server/ui/tests/test_kiosk_screens_tool.py, server/ui/tests/browser/test_kiosk_names.py, server/README.md