Fix missing http status import for PD failure handler (#6520)

Signed-off-by: Shangming Cai <caishangming@linux.alibaba.com>

python/sglang/srt/disaggregation/prefill.py

@@ -22,6 +22,7 @@ from __future__ import annotations
 import logging
 import threading
 from collections import deque
+from http import HTTPStatus
 from typing import TYPE_CHECKING, List, Optional
 
 import torch