docs(contributing): install browsers before running tests - #3199

CONTRIBUTING.md takes a contributor from pip install -e . and python -m build --wheel straight to pytest --browser chromium. The browsers are never installed, and playwright install does not appear anywhere in CONTRIBUTING.md or README.md.

CI does install them, explicitly, at three places in .github/workflows/ci.yml:

line 100:  python -m playwright install --with-deps ${{ matrix.browser }}
line 147:  python -m playwright install ${{ matrix.browser-channel }} --with-deps
line 178:  python -m playwright install --with-deps chromium

So the step is real and known, it just lives only in the workflow. Nothing in conftest.py or setup.py installs browsers on the contributor's behalf.

I used plain playwright install chromium rather than CI's --with-deps, since that flag installs system packages and wants root on Linux, which is right for a runner and wrong as the default line in a contributor doc.

Verified by reading the tree and the workflow. I did not run the suite on this machine, so I am not claiming a specific pytest error string, only that the documented path never installs what the test command needs.