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/run_slash_command.py

@@ -45,7 +45,7 @@ class RunSlashCommandTool(MoviePilotTool):
     def get_tool_message(self, **kwargs) -> Optional[str]:
         """生成友好的提示消息"""
         command = kwargs.get("command", "")
-        return f"正在执行命令: {command}"
+        return f"执行命令: {command}"
 
     async def run(self, command: str, **kwargs) -> str:
         logger.info(f"执行工具: {self.name}, 参数: command={command}")