fix(uploader): add key attribute to UploaderPreviewItem to avoid rendering issues

[ascii5]

issue: #13721

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.02%. Comparing base (ec5b45b) to head (563d27c).
⚠️ Report is 190 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/vant/src/uploader/Uploader.tsx	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13743      +/-   ##
==========================================
+ Coverage   89.60%   90.02%   +0.42%     
==========================================
  Files         257      257              
  Lines        7013     7051      +38     
  Branches     1736     1748      +12     
==========================================
+ Hits         6284     6348      +64     
+ Misses        384      366      -18     
+ Partials      345      337       -8     

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[ascii5]

更改了key测试用例无法直接拿key值测试，测试了删除中间item的行为