Improve image.go test coverage to 100%

[sashabaranov]

Summary

• rewrite image tests
• exercise all error branches in CreateEditImage and CreateVariImage
• add failing request builder cases

Testing

• go test ./... -coverprofile=coverage.out

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https://chatgpt.com/codex/tasks/task_e_6871278401f883259b6baffa0e97a8a4

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.11%. Comparing base (88eb1df) to head (a21c5d3).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1039      +/-   ##
==========================================
+ Coverage   97.15%   98.11%   +0.96%     
==========================================
  Files          34       34              
  Lines        2177     2177              
==========================================
+ Hits         2115     2136      +21     
+ Misses         43       29      -14     
+ Partials       19       12       -7     

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