fix: use WorkerPool.shutdown() in _reset_pool to avoid union-attr mypy error

_reset_pool accessed _pool._pool.shutdown() directly but mypy can't
narrow the inner ProcessPoolExecutor | None to non-None. Replaced with
a call to the public shutdown() method which already handles the check.

Author: Peter-L-SVK

src/core/worker_pool.py

@@ -300,8 +300,5 @@ def _reset_pool() -> None:
     """Force-shutdown and recreate the global worker pool."""
     global _pool
     if _pool is not None:
-        try:
-            _pool._pool.shutdown(wait=False, cancel_futures=True)
-        except Exception:
-            pass
+        _pool.shutdown()
         _pool = None