Add tests for month and year logic and improve coverage

[martijnrusschen]

Introduces new test suites for month and year logic helpers, covering keyboard navigation, disabled states, and focus management. Adds additional tests for edge cases in date utilities, click outside wrapper, shadow root, and exposes new helpers via the main entry point. Improves robustness and coverage of calendar and datepicker components.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.22%. Comparing base (00e3fb0) to head (8ce6054).
⚠️ Report is 22 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6025      +/-   ##
==========================================
+ Coverage   98.91%   99.22%   +0.31%     
==========================================
  Files          30       30              
  Lines        3496     3495       -1     
  Branches     1486     1486              
==========================================
+ Hits         3458     3468      +10     
+ Misses         37       26      -11     
  Partials        1        1              

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