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/send_local_file.py

@@ -55,7 +55,7 @@ class SendLocalFileTool(MoviePilotTool):
     def get_tool_message(self, **kwargs) -> Optional[str]:
         file_path = kwargs.get("file_path", "")
         file_name = Path(file_path).name if file_path else "未知文件"
-        return f"正在发送本地附件: {file_name}"
+        return f"发送本地附件: {file_name}"
 
     async def run(
         self,