Increase files.go test coverage

[sashabaranov]

Summary

• add a test ensuring CreateFile returns request builder errors

Testing

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

---

https://chatgpt.com/codex/tasks/task_e_68712fc6bc7c8325a8df49c2c030f7cd

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.31%. Comparing base (8665ad7) to head (63a62cd).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1041      +/-   ##
==========================================
+ Coverage   99.17%   99.31%   +0.13%     
==========================================
  Files          34       34              
  Lines        2177     2177              
==========================================
+ Hits         2159     2162       +3     
+ Misses         12       10       -2     
+ Partials        6        5       -1     

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