test: add formatWeekDay prop test for DatePicker component

[martijnrusschen]

Summary

• Adds test coverage for the formatWeekDay prop when used through the DatePicker component
• The existing test only covered the Calendar component directly
• This ensures the prop is properly passed through from DatePicker to Calendar

Test plan

• Run yarn test:ci - all 1471 tests pass
• Run yarn lint - passes
• Run yarn type-check - passes

Related

• Investigated in response to issue formatWeekDay not working on v8 or v9 (in Tauri) #6173
• The issue appears to be environment-specific (Tauri), but this test improves coverage regardless

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.31%. Comparing base (041f1ac) to head (344d1ed).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6177   +/-   ##
=======================================
  Coverage   99.31%   99.31%           
=======================================
  Files          30       30           
  Lines        3801     3801           
  Branches     1635     1652   +17     
=======================================
  Hits         3775     3775           
  Misses         25       25           
  Partials        1        1           

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