fix(terminal): strict Linux-only gating for background-executor systemd scopes (#70716 follow-up)

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.

.github/workflows/windows-venv-e2e.yml

@@ -70,3 +70,11 @@ jobs:
           uv run --no-sync python -m pytest \
             tests/gateway/test_telegram_closewait_windows_live.py \
             -o addopts= -v -p no:cacheprovider
+
+      - name: Run background-executor spawn parity live E2E (#70716)
+        shell: bash
+        run: |
+          set -uo pipefail
+          uv run --no-sync python -m pytest \
+            tests/tools/test_process_registry_windows_live.py \
+            -o addopts= -v -p no:cacheprovider