Split tests into smaller suites

[BelfordZ]

Summary

• split large index.test.ts into focused suites
• add a small test-helpers utility used across tests

Testing

• npm test

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https://chatgpt.com/codex/tasks/task_e_68551242d970832f952caad8e69410a3

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (c7796c2) to head (77b4555).
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Additional details and impacted files

@@            Coverage Diff            @@
##            master      #820   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         3    +1     
  Lines          115       123    +8     
  Branches        42        47    +5     
=========================================
+ Hits           115       123    +8     

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