fix: rename _worker_pool to _pool in _reset_pool()

- _reset_pool() in worker_pool.py referenced _worker_pool but the global
  singleton is named _pool, causing three mypy name-defined errors

Author: Peter-L-SVK

src/core/worker_pool.py

@@ -298,10 +298,10 @@ def get_pool() -> WorkerPool:
 
 def _reset_pool() -> None:
     """Force-shutdown and recreate the global worker pool."""
-    global _worker_pool
-    if _worker_pool is not None:
+    global _pool
+    if _pool is not None:
         try:
-            _worker_pool._pool.shutdown(wait=False, cancel_futures=True)
+            _pool._pool.shutdown(wait=False, cancel_futures=True)
         except Exception:
             pass
-        _worker_pool = None
+        _pool = None