The new standalone test file duplicated TestGitBashExternalProgramProbe's
harness; one assertion on the recorded kwargs covers the #78820 contract.
The sibling ASLR-probe assertion is dropped (unchanged code, already green
on main). Comment trimmed to the WHY.
`_bash_starts()` in tools/environments/local_gitbash_probe.py ran bash.exe
with capture_output=True but no stdin=, so the MSYS2 child inherited the
TUI gateway's stdin pipe. The MSYS2 runtime switches that shared pipe to
PIPE_NOWAIT; the gateway's next sys.stdin.readline() then fails with
ERROR_NO_DATA, which the CRT maps to OSError(EINVAL), and the gateway
exits with code 1 ("gateway exited") on the first terminal call.
The sibling probe _mandatory_aslr_enabled() already passes
stdin=subprocess.DEVNULL; this brings _bash_starts() in line with it.
Add tests/tools/test_gitbash_probe_stdin.py asserting both probes forward
stdin=DEVNULL to subprocess.run (fails on the pre-fix source).
Repo scanners (check_subprocess_stdin, check-windows-footguns --all) flagged 21 sites where
the r3 single-line collapses lost stdin=DEVNULL, encoding='utf-8'/errors='replace', the
'# windows-footgun: ok' same-line marker, or the getattr(os, 'geteuid') gate. Each guard is
restored at the call site (real portability/hang fixes, not suppressions).
