Image Health Check fails during pytest plugin loading #1241

All 22 Image Health Check matrix jobs fail before test collection with ModuleNotFoundError: No module named 'platformdirs.pytest_plugin', reproduced on the rerun of https://github.com/citusdata/packaging/actions/runs/36956957513. The newer runner environment appears to mix ambient pytest plugin metadata with pinned platformdirs==4.2.2; the exact ambient metadata source is not confirmed.

#1240 isolates dependency installation and pytest execution in a venv while preserving the existing tools and dependency pins.