fix: Correctly format message parts in GoogleChat (#8596)

### What problem does this PR solve?

This PR addresses an incompatibility issue with the Google Chat API by
correcting the message content format in the `GoogleChat` class.
Previously, the content was directly assigned to the "parts" field,
which did not align with the API's expected format. This change ensures
that messages are properly formatted with a "text" key within a
dictionary, as required by the API.

### Type of change

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

rag/llm/chat_model.py

@@ -1596,7 +1596,9 @@ class GoogleChat(Base):
             if "role" in item and item["role"] == "assistant":
                 item["role"] = "model"
             if "content" in item:
-                item["parts"] = item.pop("content")
+                item["parts"] = [{
+                    "text": item.pop("content"),
+                }]
 
         response = self.client.generate_content(hist, generation_config=gen_conf)
         ans = response.text