fix(189pc): sliceSize must not be equal to fileSize

[xrgzs]

Description / 描述

1. 将原来的 size 变量改为更具体的 fileSize，提升代码可读性。
2. 修正了 sliceSize 的计算方式，确保分片大小不会等于文件大小，避免上传异常。
3. 相关分片处理逻辑（如分片计数、最后一片大小、分片读取等）全部用更明确的变量名和逻辑替换，注释也更清晰。
4. 保证 sliceSize 参数始终为特定分片大小，避免与文件大小混淆。

Motivation and Context / 背景

Closes #1126

How Has This Been Tested? / 测试

Checklist / 检查清单

• I have read the CONTRIBUTING document.
  我已阅读 CONTRIBUTING 文档。
• I have formatted my code with go fmt or prettier.
  我已使用 go fmt 或 prettier 格式化提交的代码。
• I have added appropriate labels to this PR (or mentioned needed labels in the description if lacking permissions).
  我已为此 PR 添加了适当的标签（如无权限或需要的标签不存在，请在描述中说明，管理员将后续处理）。
• I have requested review from relevant code authors using the "Request review" feature when applicable.
  我已在适当情况下使用"Request review"功能请求相关代码作者进行审查。
• I have updated the repository accordingly (If it’s needed).
  我已相应更新了相关仓库（若适用）。
  • OpenList-Frontend #XXXX
  • OpenList-Docs #XXXX

[Copilot]

Pull Request Overview

This PR fixes an issue where sliceSize could equal fileSize causing upload failures. The changes improve code readability by using more descriptive variable names and ensure proper slice size calculation for file uploads.

• Rename size variable to more descriptive fileSize throughout the upload logic
• Modify sliceSize calculation to ensure it's never equal to file size
• Update all related slice processing logic to use clearer variable names and comments

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