fix: improve DatePickerProps discriminated union type inference

[martijnrusschen]

This fix addresses issue #5391 where TypeScript was not properly inferring the onChange callback type based on selectsRange and selectsMultiple props.

The problem was that using 'never' as the discriminant type for optional properties didn't allow TypeScript to properly narrow the union type.

Changes:

• Changed selectsRange?: never to selectsRange?: false | undefined
• Changed selectsMultiple?: never to selectsMultiple?: false | undefined
• Updated test file to use correct type for selectsMultiple prop

This allows TypeScript to correctly infer:

• onChange receives Date | null when selectsRange is false/undefined
• onChange receives [Date | null, Date | null] when selectsRange is true
• onChange receives Date[] | null when selectsMultiple is true

Fixes #5391

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.22%. Comparing base (f318d4b) to head (b38a660).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6078      +/-   ##
==========================================
+ Coverage   99.14%   99.22%   +0.08%     
==========================================
  Files          30       30              
  Lines        3612     3612              
  Branches     1551     1547       -4     
==========================================
+ Hits         3581     3584       +3     
+ Misses         29       27       -2     
+ Partials        2        1       -1     

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