test(tools): make subprocess env regressions Windows-portable

The PYTHONPATH/PATH sanitization suite was written POSIX-centric and
failed on real Windows 11 (reproduced natively: 4 failures before this
change).  Fix the tests to express the true per-platform contract:

- test_other_major_version_site_packages_preserved /
  test_make_run_env_injects_hermes_bin_dir: build inputs with
  os.pathsep instead of hardcoded ':'.
- test_make_run_env_appends_homebrew_on_minimal_path: split on
  os.pathsep, neutralise Git Bash dir prepending, and assert the
  documented Windows passthrough (_append_missing_sane_path_entries is
  a no-op off POSIX) instead of the Homebrew append.
- test_make_run_env_real_launchd_path_gains_homebrew: mark
  macos_only per repo OS-marker policy (the regression is the macOS
  launchd PATH; the merge is a passthrough on Windows).
- test_configured_home_alias_matches_launcher_output: create the
  configured-home link via a helper that falls back to an unprivileged
  directory junction (cmd /c mklink /J) when symlink creation raises
  WinError 1314, and skips with a clear reason if no mechanism exists.

Also correct a stale comment in execute_code: the child is not always
the same Python as Hermes (project mode can select an external venv),
so the strip is about compatibility, not redundancy.

tools/code_execution_tool.py

@@ -1486,10 +1486,10 @@ def execute_code(
         #
         # Before re-injecting PYTHONPATH, strip Hermes-owned entries that
         # leaked through _scrub_child_env (PYTHONPATH is in _SAFE_ENV_PREFIXES
-        # so it passes the scrub).  The sandbox runs the SAME Python as
-        # Hermes, so the Hermes venv entries are redundant — and if they
-        # came from a different Hermes venv they would poison the sandbox's
-        # sys.path with ABI-incompatible C extensions (#74817).
+        # so it passes the scrub).  They are redundant for same-Hermes-
+        # environment children and may be incompatible with external
+        # interpreters (project mode can select a different venv), so they
+        # must not shadow or poison the child's sys.path (#74817).
         from tools.environments.local import _strip_hermes_owned_pythonpath
         _strip_hermes_owned_pythonpath(child_env)
         _hermes_root = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))