fix: update SystemMessage content assignment in base_chat_step

--bug=1069054@tapd-62980211 --user=刘瑞斌 【简易应用】长期记忆功能中提示词{memory}没有被替换成记忆 https://www.tapd.cn/62980211/s/1916885

Author: liuruibin

apps/application/chat_pipeline/step/chat_step/impl/base_chat_step.py

@@ -392,6 +392,7 @@ def get_stream_result(self, message_list: List[BaseMessage],
                 if isinstance(msg, SystemMessage):
                     if isinstance(msg.content, str):
                         user_system_prompt = msg.content.replace('{memory}', memory)
+                        msg.content = user_system_prompt
                     elif isinstance(msg.content, list):
                         user_system_prompt = ''.join(
                             item.get('text', '') if isinstance(item, dict) else str(item)