[Bug]: 假多线程

### Installation Method | 安装方法与平台

Pip Install (I used latest requirements.txt)

### Version | 版本

Others | 非最新版

### OS | 操作系统

Linux

### Describe the bug | 简述

在执行“精细翻译PDF论文”任务时，表面上启动了多线程工作，但实际上只有一个线程真正在执行

### Screen Shot | 有帮助的截图


### Terminal Traceback & Material to Help Reproduce Bugs | 终端traceback（如有） + 帮助我们复现的测试材料样本（如有）

_No response_

The report identifies the fine-grained PDF paper translation task but names no source file, test, version, or reproducible sample. Start by locating that task in the Python project and checking whether its apparent multithreading performs work concurrently on Linux. Done means reproducing the single-worker behavior, identifying its cause, and adding a regression check if the relevant test location can be found.

Written by the indexing model from the issue text.