Fix: Raptor: [Bug]: **ERROR**: Unknown field for GenerationConfig: max_tokens (#8331)

### What problem does this PR solve?

https://github.com/infiniflow/ragflow/issues/8324

### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)

rag/llm/chat_model.py

 
     def _clean_conf(self, gen_conf):
         for k in list(gen_conf.keys()):
-            if k not in ["temperature", "top_p", "max_tokens"]:
+            if k not in ["temperature", "top_p"]:
                 del gen_conf[k]
         return gen_conf