关于adk/middlewares/summarization/summarization.go中总结时的输入可能超出模型上下文窗口的问题

需要处理默认情况下用户消息超出了总结模型上下文窗口的问题：

可以考虑默认情况下使用触发阈值作为限制：滑动窗口策略淘汰旧消息，避免用于总结的消息超过了总结模型本身的上下文限制

Start by reading adk/middlewares/summarization/summarization.go and trace how messages are selected before they are sent to the summarization model. Define the default behavior so the input stays within the trigger threshold, using a sliding window to discard older messages, and verify that oversized inputs no longer exceed the model context window.

Written by the indexing model from the issue text.