tests(cli[sync_sigint]) Anchor subprocess to vcspull's resolved location

[tony]

Summary

• Fix a test failure for distros (e.g. Arch Linux) that run vcspull's test suite against a staged-install directory referenced via a relative PYTHONPATH entry.
• Anchor the subprocess spawned by test_exit_on_sigint_produces_wifsignaled_sigint to vcspull's actual install location via importlib.util.find_spec, then prepend that absolute directory to the child's PYTHONPATH.
• Preserve the original PYTHONPATH so other build environments remain unaffected.

Root cause

Distros stage built wheels into a tree like tmp_install/usr/lib/pythonX.Y/site-packages/ and run pytest from the source dir with a relative PYTHONPATH=tmp_install/usr/lib/pythonX.Y/site-packages:. The project's root conftest.py has an autouse cwd_default fixture that monkeypatch.chdir(tmp_path)s every test. When test_exit_on_sigint_produces_wifsignaled_sigint calls subprocess.run([sys.executable, "-c", runner], env={**os.environ, ...}), the child inherits the per-test tmp_path as its CWD and the relative PYTHONPATH. The child resolves the relative entry against the tmp dir, finds nothing, and from vcspull.cli.sync import _exit_on_sigint fails with ModuleNotFoundError: No module named 'vcspull'. uv run pytest masks the bug because the venv exposes vcspull via absolute site-packages.

Reproduced locally by staging the built wheel into ./tmp_install/... and running PYTHONNOUSERSITE=1 PYTHONPATH="\$STAGE:" python3.14 -s -m pytest tests/cli/test_sync_sigint.py — yields the same stderr as the original failure.

Test plan

• `uv run pytest tests/cli/test_sync_sigint.py -v` — the target test passes in the dev env
• `PYTHONNOUSERSITE=1 PYTHONPATH="$STAGE:" python3.14 -s -m pytest tests/cli/test_sync_sigint.py` — same test passes under a relative-`PYTHONPATH` staged-install setup (previously failed with `ModuleNotFoundError`)
• `uv run pytest` — full suite passes
• `uv run ruff format .` — clean
• `uv run ruff check .` — clean
• `uv run mypy` — clean

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.61%. Comparing base (833f234) to head (ffd1308).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #549   +/-   ##
=======================================
  Coverage   83.61%   83.61%           
=======================================
  Files          29       29           
  Lines        4114     4114           
  Branches      813      813           
=======================================
  Hits         3440     3440           
  Misses        440      440           
  Partials      234      234           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.