ci: use uv.lock when running tests

[keturn]

Summary

Given we run a "uv lock checks" workflow, I assume that means we intend uv.lock to be used.

The uv pip interface is a different thing than the rest of uv (sync, run, etc), it doesn't use uv.lock (but can use version constraints expressed in other ways). Use uv sync instead.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

[keturn]

All Checks Have Passed
python tests / py3.11: linux-cpu (pull_request) Successful in 31s

This is misleading because this doesn't change a python file, so it didn't actually run the python tests.