stop_str of qwen2-vl template should be a tuple not a str (#1834)

Co-authored-by: Byron Hsu <byronhsu1230@gmail.com>

python/sglang/lang/chat_template.py

@@ -144,7 +144,7 @@ register_chat_template(
             "assistant": ("<|im_start|>assistant\n", "<|im_end|>\n"),
         },
         style=ChatTemplateStyle.PLAIN,
-        stop_str=("<|im_end|>"),
+        stop_str=("<|im_end|>",),
         image_token="<|vision_start|><|image_pad|><|vision_end|>",
     )
 )