fix: Windows subprocess exit-code normalization

[jbeckwith-oai]

Summary

Normalize Windows subprocess failure statuses before exiting the Python wrapper so missing-config errors produce a stable process exit code instead of unsigned DWORD/NTSTATUS values.

Root cause

Recent Windows CI runs on main and PR 34 started reporting config-error smoke test exit codes like 4294967295 and 3221226505 from the wrapper's sys.exit(result.returncode) path. Those values correspond to Windows-specific failure statuses rather than the expected shell-style 1 for config errors.

Validation

• env -u VIRTUAL_ENV uv sync --extra dev --frozen
• env -u VIRTUAL_ENV uv run --frozen pytest tests/test_cli.py -v
• env -u VIRTUAL_ENV uv run --frozen pytest tests/smoke/test_smoke.py -k test_config_error_exit_code -v

Notes

This is intentionally separate from Dependabot PR #34 and targets main directly. Once CI is green here, PR #34 can be rerun or rebased.

[mldangelo-oai]

Reviewed the exit-code normalization path and tests. This fixes the Windows smoke regression by collapsing unsigned/NTSTATUS-style failures to exit code 1 while preserving normal CLI codes like 0/1/100, and the POSIX signal mapping to 128+signal is a good portability improvement over passing negative codes to sys.exit(). CI is green across Linux/Windows and the added unit coverage hits both the helper and main(). One non-blocking note: Windows statuses with the sign bit set are intentionally lossy now, but that seems like the right tradeoff for promptfoo’s documented low-range exit codes.