Fix:Error when parsing files using Gemini: **ERROR**: GENERIC_ERROR - Unknown field for GenerationConfig: max_tokens (#9195)

### What problem does this PR solve?
https://github.com/infiniflow/ragflow/issues/9177
The reason should be due to the gemin internal use a different parameter
name
`
        max_output_tokens (int):
            Optional. The maximum number of tokens to include in a
            response candidate.

            Note: The default value varies by model, see the
            ``Model.output_token_limit`` attribute of the ``Model``
            returned from the ``getModel`` function.

            This field is a member of `oneof`_ ``_max_output_tokens``.
`
### Type of change

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

rag/llm/chat_model.py

         for k in list(gen_conf.keys()):
             if k not in ["temperature", "top_p", "max_tokens"]:
                 del gen_conf[k]
+            # if max_tokens exists, rename it to max_output_tokens to match Gemini's API
+            if k == "max_tokens":
+                gen_conf["max_output_tokens"] = gen_conf.pop("max_tokens")
         return gen_conf
 
     def _chat(self, history, gen_conf={}, **kwargs):