Fix: OAuth token save crashes on Windows (os.fchmod is Unix-only)

[SENTMarketing]

Problem

_save_oauth_token() in scripts/google_auth.py calls os.fchmod(fd, 0o600). On Windows os.fchmod does not exist, so the call raises AttributeError — which is not caught by the surrounding except OSError. The exception propagates before os.fdopen(...).write() runs, leaving an empty oauth-token.json and making --auth fail on Windows.

Observed error during google_auth.py --auth:

Error exchanging authorization code: module 'os' has no attribute 'fchmod'

(token file ends up empty; refresh/login broken on Windows.)

Fix

Guard the call with hasattr(os, "fchmod") so non-POSIX platforms simply skip it and proceed to write the token. POSIX behaviour is unchanged (fchmod still enforces 0o600), and _chmod_quiet() already set 0o600 on the file beforehand.

try:
    if hasattr(os, "fchmod"):  # fchmod is Unix-only; absent on Windows
        os.fchmod(fd, 0o600)
except OSError:
    pass

Verified: --auth now writes a valid token with refresh_token on Windows 11 (Python 3.12); unchanged on POSIX.