bugfix: Prevent global mutation of conv.stop_str across requests (#7347)

Co-authored-by: Chang Su <chang.s.su@oracle.com>

python/sglang/srt/entrypoints/openai/serving_chat.py

+import copy
 import json
 import logging
 import time
@@ -292,7 +293,7 @@ class OpenAIServingChat(OpenAIServingBase):
         image_data = conv.image_data if conv.image_data else None
         audio_data = conv.audio_data if conv.audio_data else None
         modalities = conv.modalities if conv.modalities else []
-        stop = conv.stop_str or [] if not request.ignore_eos else []
+        stop = copy.copy(conv.stop_str or [] if not request.ignore_eos else [])
 
         if request.stop:
             if isinstance(request.stop, str):