fix: restore Path import in env_passthrough.py (removed by #5526)

The ContextVar migration removed 'from pathlib import Path' but Path
is still used in _load_config_passthrough(). Without this import,
config-based env passthrough would raise NameError.

tools/env_passthrough.py

@@ -22,6 +22,7 @@ from __future__ import annotations
 import logging
 import os
 from contextvars import ContextVar
+from pathlib import Path
 from typing import Iterable
 
 logger = logging.getLogger(__name__)