gapfill G1: remove 5 ruff F401 unused imports (3 unused os in production files, 2 in tests); no assert lines changed, ruff all cloud now 0

cloud/marker_tools.py

@@ -131,7 +131,6 @@ import。DB 助手函数只用 ? 占位符，与 peer_messages 同款写法。
 """
 
 import json
-import os
 import re
 import time
 import uuid