Cross-platform hardening of @toprakeker's systemd cgroup isolation
(PR #71378, landed via #81264):
- Gate every scope-path branch on a new _IS_LINUX constant instead of
'not _IS_WINDOWS', so macOS (and any other POSIX platform) provably
never touches systemd code — no probe subprocess, no scope argv,
byte-identical legacy spawn.
- Unit tests: darwin no-op guarantee (no probe exec, no scope argv build,
legacy argv byte-identical, no unit recorded) and probe-returns-False
off Linux.
- New live Windows E2E (tests/tools/test_process_registry_windows_live.py,
wired into the on-demand windows-venv-e2e lane): real spawn_local on
windows-latest asserting jobs run exactly as before — spawned, output
captured, exit code correct, systemd path never reached even under
faked gateway identity.
Refs #70716, #71378.
Teknium
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