[Bug]:QwenPaw 不报空响应错误

## QwenPaw Version

Windows 11,2.01

## Description

QwenPaw 不报空响应错误：长会话可能因正常工具调用累积而逐渐逼近窗口上限。到那时模型仍会空响应，QwenPaw 仍不报错。这是框架层问题。
它导致了长下文的会话中，使用某些模型时会话中彻底失去响应。

Start by trying to reproduce the reported QwenPaw 2.01 behavior in a long conversation with normal tool calls until the context window is nearly full. Then inspect the framework-level model response handling path for empty responses. Done means an empty model response is reported as an error instead of leaving the conversation with no response.

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