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

tools/mcp_oauth.py

@@ -48,6 +48,7 @@ from http.server import BaseHTTPRequestHandler, HTTPServer
 from pathlib import Path
 from typing import Any
 from urllib.parse import parse_qs, urlparse
+from hermes_constants import secure_parent_dir
 
 logger = logging.getLogger(__name__)
 
@@ -175,10 +176,8 @@ def _write_json(path: Path, data: dict) -> None:
     path.parent.mkdir(parents=True, exist_ok=True)
     # Tighten parent dir to 0o700 so siblings can't traverse to the creds.
     # No-op on Windows (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)
     # Per-process random suffix avoids collisions between concurrent
     # writers and stale leftovers from a prior crashed write.
     tmp = path.with_suffix(f".tmp.{os.getpid()}.{secrets.token_hex(4)}")