fix(security): guard os.chmod(parent) against / and top-level dirs

Five call sites do os.chmod(path.parent, 0o700) without checking that
the parent resolves to a safe directory. If HERMES_HOME or another
path env var resolves to /, the chmod strips traversal permission from
the root inode and bricks the entire host.

Add secure_parent_dir() to hermes_constants.py that refuses to chmod
/ or any top-level directory (depth < 2). Replace all 5 call sites
with this helper.

Fixes #25821

agent/google_oauth.py

@@ -59,7 +59,7 @@ from dataclasses import dataclass
 from pathlib import Path
 from typing import Any, Dict, Optional, Tuple
 
-from hermes_constants import get_hermes_home
+from hermes_constants import get_hermes_home, secure_parent_dir
 
 logger = logging.getLogger(__name__)
 
@@ -491,10 +491,8 @@ def save_credentials(creds: GoogleCredentials) -> Path:
     path.parent.mkdir(parents=True, exist_ok=True)
     # Tighten parent dir to 0o700 so siblings can't traverse to the creds file.
     # On Windows this is a no-op (POSIX mode bits aren't enforced); ignore failures.
-    try:
-        os.chmod(path.parent, 0o700)
-    except OSError:
-        pass
+    # secure_parent_dir refuses to chmod / or top-level dirs (#25821).
+    secure_parent_dir(path)
     payload = json.dumps(creds.to_dict(), indent=2, sort_keys=True) + "\n"
 
     with _credentials_lock():