使用brpc ProgressiveReader 接口请求server异常情况阻塞

**Describe the bug (描述bug)**
最新版brpc，使用 OnReadOnePart 接口请求server，server端流式返回数据，回传数据中 server 挂掉，未调用OnEndOfMessage，接口阻塞

**Expected behavior (期望行为)**
期望在server挂掉时，可以捕获异常，正常结束本次 ProgressiveReader 调用

Start by locating the ProgressiveReader implementation and the OnReadOnePart and OnEndOfMessage handling paths. Reproduce a streaming response where the server stops before OnEndOfMessage, then trace how the disconnect is propagated; done means the call detects the server failure and returns instead of blocking.

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