fix: add on chat start callback impl (#662)

Author: pawel-kotowski

src/rai_core/rai/agents/integrations/streamlit.py

@@ -74,6 +74,21 @@ def on_llm_new_token(self, token: str, **kwargs) -> None:
             self.text += token  # Append the new token to the existing text
             self.token_placeholder.write(self.text)
 
+        def on_chat_model_start(
+            self,
+            serialized: Dict[str, Any],
+            messages: List[List[BaseMessage]],
+            **kwargs: Any,
+        ) -> None:
+            """
+            Run when the chat model starts.
+            Args:
+                serialized (Dict[str, Any]): The serialized model.
+                messages (List[List[BaseMessage]]): The messages.
+                **kwargs: Additional keyword arguments.
+            """
+            pass  # No specific action needed for chat model start
+
         def on_tool_start(
             self, serialized: Dict[str, Any], input_str: str, **kwargs: Any
         ) -> None: