fix: improve DatePickerProps discriminated union type inference

[martijnrusschen]

This fix addresses issues #5391 and #5198 where TypeScript was not properly inferring types 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
Fixes #5198

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.14%. Comparing base (f318d4b) to head (c154ff9).
⚠️ Report is 36 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6100   +/-   ##
=======================================
  Coverage   99.14%   99.14%           
=======================================
  Files          30       30           
  Lines        3612     3620    +8     
  Branches     1551     1555    +4     
=======================================
+ Hits         3581     3589    +8     
  Misses         29       29           
  Partials        2        2           

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