* bugfix #5179
* test: cover the env-aware _pick_python fallback from #5179
Follow the test_serve_nginx_stop.py extraction pattern: drive the real
_pick_python from serve.sh against a stub-only PATH plus a mocked env.
- python3 succeeds directly but fails through env -> python selected
(red on main, green on this branch)
- env rejects every candidate -> nonzero exit (also red on main)
- healthy PATH with the real env -> python3 preferred, guarding against
over-rejection
MSYS/Git Bash hosts need the stub dir as an MSYS-style (/c/...) PATH
entry, and bash diagnostics may arrive in the console code page, so the
runner decodes output with errors="replace".
