fix: prevent file processing crash and duplicate on long text paste

[chengcheng84]

Fix #314 #235

修复之后发现，文本会被粘贴为2个文件

[eeee0717]

Potential race in processingTextRef: each async setText call always clears the ref in finally. If long text A starts, then long text B starts, A can finish first and set the ref to null while B is still processing. A subsequent setText(B) bypasses dedupe and may trigger duplicate conversion/file creation. Suggestion: only clear when processingTextRef.current === newText (or use a monotonic request id).

[eeee0717]

可按下面方式修复，避免旧请求把新请求的 guard 清掉：

} finally {
  if (processingTextRef.current === newText) {
    processingTextRef.current = null
  }
}

这样在 A、B 并发时，只有仍持有当前 guard 的那次调用才会清空，setText(B) 不会在 B 处理中被误放行。