Update chat_model.py (#9318)

### What problem does this PR solve?
https://github.com/infiniflow/ragflow/issues/9317
base on
https://discuss.ai.google.dev/t/valueerror-invalid-operation-the-response-text-quick-accessor-requires-the-response-to-contain-a-valid-part-but-none-were-returned/42866
should can be handled by retry 
### Type of change

- [x] Refactoring

rag/llm/chat_model.py

@@ -1099,9 +1099,20 @@ class GeminiChat(Base):
 
         if system:
             self.model._system_instruction = content_types.to_content(system)
-        response = self.model.generate_content(hist, generation_config=gen_conf)
+        retry_count = 0
-        ans = response.text
+        max_retries = 3
-        return ans, response.usage_metadata.total_token_count
+        while retry_count < max_retries:
+            try:
+                response = self.model.generate_content(hist, generation_config=gen_conf)
+                ans = response.text
+                return ans, response.usage_metadata.total_token_count
+            except Exception as e:
+                retry_count += 1
+                if retry_count >= max_retries:
+                    raise e
+                else:
+                    import time
+                    time.sleep(50) 
 
     def chat_streamly(self, system, history, gen_conf={}, **kwargs):
         from google.generativeai.types import content_types