feat(fill): watch mode with re-run for `uv run fill`

[raxhvl]

I'm iterating on a new test using uv run fill, and it doesn’t pass yet. To speed up development, I’d like to automatically re-run the command whenever files in tests/ change, similar to watch modes in other test runners.

Current Workarounds

Using watch:

watch uv run fill

@danceratopz has ideas to tackle this.

Some references

• https://github.com/olzhasar/pytest-watcher
• https://github.com/eradman/entr

[danceratopz]

The currently available cli option --looponfail is a deprecated[1] feature[2] from pytest-xdist. This runs continuously and throttles CPU, a known bug that won't get fixed as its deprecated. I got this working after some initial errors, but I don't think this is the correct solution (migration path to pytest core is unclear[3]).

I think we need to go with one of the following:

1. https://github.com/joeyespo/pytest-watch
2. https://github.com/olzhasar/pytest-watcher

And (edit; added later), we should check whether https://testmon.org/ helps/improves this functionality. From the website:

If you want to run your tests on every file save we recommend using testmon with pytest-watch.

Testmon always re-executes tests which failed last time.

[1] pytest-dev/pytest-xdist#826
[2] https://pytest-xdist.readthedocs.io/en/stable/index.html#features
[3] pytest-dev/pytest-xdist#826 (comment)

[danceratopz]

Another, but less less-generic, way to solution is to use this VS Code extension:
https://marketplace.visualstudio.com/items?itemName=LittleFoxTeam.vscode-python-test-adapter