refine tool提示语为更简洁风格，补充last_buffer_char属性及非VERBOSE模式流式输出换行逻辑，新增工具流式分隔符单元测试

app/agent/tools/impl/delete_transfer_history.py

@@ -30,7 +30,7 @@ class DeleteTransferHistoryTool(MoviePilotTool):
     def get_tool_message(self, **kwargs) -> Optional[str]:
         """根据参数生成友好的提示消息"""
         history_id = kwargs.get("history_id")
-        return f"正在删除整理历史记录: ID={history_id}"
+        return f"删除整理历史记录: ID={history_id}"
 
     async def run(self, history_id: int, **kwargs) -> str:
         logger.info(f"执行工具: {self.name}, 参数: history_id={history_id}")