- emit STOPPED terminal chunk on abort so partial text and pending
  auditResult/auditExport stay attached to the message
- fill '已停止生成。' when nothing was streamed, and skip local
  fallback scoring once the user has stopped
- treat aborted signal (including string abort reason) as terminal in
  the request catch
- drop stale stream/error updates in useChat when the target message
  was removed, e.g. after switching conversations