fix(computer-use): restore subprocess import lost in upstream merge

cua_driver_update_check() calls subprocess.run but the import was dropped
when the PR branch merged main's unused-import prune (66827f894). The
try/except Exception swallowed the NameError, silently disabling the
update check.

tools/computer_use/cua_backend.py

@@ -40,6 +40,7 @@ import logging
 import os
 import re
 import shutil
+import subprocess
 import sys
 import threading
 from typing import Any, Dict, List, Optional, Tuple