Increase jsonschema test coverage

[sashabaranov]

Summary

• add tests for error branches in jsonschema.GenerateSchemaForType
• cover containsRef recursion logic

Testing

• go test ./jsonschema -cover
• go test ./...

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https://chatgpt.com/codex/tasks/task_e_68713954b31483259e4f1e24fc78d4cb

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.59%. Comparing base (bd36c45) to head (fa9e9a0).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1043      +/-   ##
==========================================
+ Coverage   99.31%   99.59%   +0.27%     
==========================================
  Files          34       34              
  Lines        2205     2205              
==========================================
+ Hits         2190     2196       +6     
+ Misses         10        6       -4     
+ Partials        5        3       -2     

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