fix(security): close TOCTOU window when saving Claude Code OAuth credentials (#21152)

_write_claude_code_credentials wrote ~/.claude/.credentials.json via
Path.write_text + replace + post-write chmod(0o600). Both the temp file
and the destination briefly inherited the process umask (commonly 0o644
= world-readable) between create/replace and chmod, exposing the OAuth
access/refresh tokens to other local users on multi-user hosts.

Use os.open with O_WRONLY|O_CREAT|O_EXCL and an explicit S_IRUSR|S_IWUSR
mode so the temp file is created atomically at 0o600. After os.replace,
the destination inherits the temp's mode, so the post-write chmod is no
longer needed. The temp name also gains a per-process random suffix to
avoid collisions between concurrent writers and stale leftovers from a
crashed prior write.

Parent dir (~/.claude/) is owned by Claude Code itself and shared with
its native auth, so we deliberately don't tighten its mode here (unlike
the mcp_oauth fix which owns its own subtree under HERMES_HOME).

Mirrors the fix shipped for agent/google_oauth.py in #19673 and the
parallel fix for tools/mcp_oauth.py in #21148.

Adds a regression test in TestWriteClaudeCodeCredentials asserting the
resulting file mode is 0o600 (skipped on Windows where POSIX mode bits
aren't enforced).

agent/anthropic_adapter.py

@@ -15,6 +15,8 @@ import json
 import logging
 import os
 import platform
+import secrets
+import stat
 import subprocess
 from pathlib import Path
 from urllib.parse import urlparse
@@ -1040,11 +1042,34 @@ def _write_claude_code_credentials(
         existing["claudeAiOauth"] = oauth_data
 
         cred_path.parent.mkdir(parents=True, exist_ok=True)
-        _tmp_cred = cred_path.with_suffix(".tmp")
-        _tmp_cred.write_text(json.dumps(existing, indent=2), encoding="utf-8")
-        _tmp_cred.replace(cred_path)
-        # Restrict permissions (credentials file)
-        cred_path.chmod(0o600)
+        # Per-process random suffix avoids collisions between concurrent
+        # writers and stale leftovers from a prior crashed write.
+        _tmp_cred = cred_path.with_suffix(f".tmp.{os.getpid()}.{secrets.token_hex(4)}")
+        try:
+            # Create the temp file atomically at 0o600. The previous
+            # write_text + post-replace chmod opened a TOCTOU window where
+            # both the temp file and the destination briefly inherited the
+            # process umask (commonly 0o644 = world-readable), exposing
+            # Claude Code OAuth tokens to other local users between create
+            # and chmod. Mirrors agent/google_oauth.py (#19673) and
+            # tools/mcp_oauth.py (#21148). Parent dir (~/.claude/) is
+            # owned by Claude Code itself, so we leave its mode alone.
+            fd = os.open(
+                str(_tmp_cred),
+                os.O_WRONLY | os.O_CREAT | os.O_EXCL,
+                stat.S_IRUSR | stat.S_IWUSR,
+            )
+            with os.fdopen(fd, "w", encoding="utf-8") as fh:
+                json.dump(existing, fh, indent=2)
+                fh.flush()
+                os.fsync(fh.fileno())
+            os.replace(_tmp_cred, cred_path)
+        except OSError:
+            try:
+                _tmp_cred.unlink(missing_ok=True)
+            except OSError:
+                pass
+            raise
     except (OSError, IOError) as e:
         logger.debug("Failed to write refreshed credentials: %s", e)