ci: run pytest via uv run to fix venv PATH breakage (#9286)

PR #9214 switched dependency installation from 'uv pip install' (which
honored UV_SYSTEM_PYTHON=1 and installed into the system Python) to
'uv sync', which always installs into a project-local .venv. The bare
'pytest' invocation in the next step never saw that venv, failing with
'pytest: command not found' (exit 127) on every matrix job.

- Run pytest via 'uv run --no-sync pytest'. The --no-sync flag is
  required: a plain 'uv run' would re-sync without '--extra test' and
  uninstall pytest before running it.
- Drop UV_SYSTEM_PYTHON=1, which is dead config now that nothing uses
  the 'uv pip' interface.
- Drop the actions/setup-python step; setup-uv is already given the
  matrix python-version and uv sync provisions its own interpreter.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>