fix: should return a invalid request response when schema missing (#9461)

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

@@ -81,6 +81,11 @@ class OpenAIServingChat(OpenAIServingBase):
                 f"This model supports at most {server_context_length} completion tokens."
             )
 
+        if request.response_format and request.response_format.type == "json_schema":
+            schema = getattr(request.response_format.json_schema, "schema_", None)
+            if schema is None:
+                return "schema_ is required for json_schema response format request."
+
         return None
 
     def _convert_to_internal_request(