fix(install): clear venv python locks on Windows

[LeonSGP43]

Summary

• stop python.exe and pythonw.exe only when they are still running from the old venv\Scripts directory during venv recreation
• keep the existing hermes.exe taskkill path and run the python cleanup before Remove-Item venv
• add a regression test that locks this install.ps1 ordering contract

Testing

• git diff --check
• python3 -m pytest tests/test_install_unmerged_index.py -q
• uv run --frozen ruff check tests/test_install_unmerged_index.py

Closes #47557

[alt-glitch]

Duplicate of #46726 — both fix scripts/install.ps1 so venv recreation stops the venv-resident pythonw.exe/python.exe (gateway/agent launched straight out of venv\Scripts\) holding .pyd locks, before Remove-Item venv. #46726 is the earlier open PR and uses the same mechanism; this one additionally adds a path-prefix guard and an ordering regression test. Both target #47557 (and follow merged #45120, which only killed hermes.exe). Suggest consolidating into #46726.