Adds TestPosixEquivalence to test_code_execution_windows_env.py. The
class pins the invariant that _scrub_child_env(env, is_windows=False)
produces byte-for-byte identical output to the pre-refactor inline
scrubber, across a matrix of:
- 2 synthetic envs (POSIX-shaped, Windows-shaped-on-POSIX)
- 3 passthrough rules (none, single-var, everything)
- 1 real-os.environ check on whatever platform runs the test
Plus a superset sanity check: is_windows=True must keep everything
is_windows=False keeps, and any extras must come from the
_WINDOWS_ESSENTIAL_ENV_VARS allowlist.
Rationale: the previous commit refactored the env-scrubbing inline
block into a helper. Future changes to that helper must not silently
regress POSIX behavior — if someone needs to change it, they update
_legacy_posix_scrubber in lockstep so the churn is visible in review.
All 21 tests in the file pass locally on Windows (pytest 9.0.3). 8 of
them are parametrized equivalence checks that run on every OS.
3b9cd58208