[release-2.19] fix: skip thumbnail generation for GIF images

[halo-dev-bot]

This is an automated cherry-pick of #6597

/assign guqing

修复 GIF 缩略图生成只会保留第一帧的问题

[ruibaby]

/lgtm

[f2c-ci-robot]

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

Attention: Patch coverage is 14.28571% with 6 lines in your changes missing coverage. Please review.

Please upload report for BASE (release-2.19@a45ebb0). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...n/halo/app/core/attachment/ThumbnailGenerator.java	14.28%	6 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##             release-2.19    #6609   +/-   ##
===============================================
  Coverage                ?   58.05%           
  Complexity              ?     3917           
===============================================
  Files                   ?      670           
  Lines                   ?    22966           
  Branches                ?     1575           
===============================================
  Hits                    ?    13334           
  Misses                  ?     9015           
  Partials                ?      617           

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