test: add tests for openToDate with showTimeSelectOnly

[martijnrusschen]

Summary

This PR adds tests to document the existing behavior that openToDate can be used to specify the default date when showTimeSelectOnly is true and selected is null.

Background

Issue #6032 requested a way to specify a default date when using showTimeSelectOnly with a null selected value. The user wanted to set a specific date (e.g., 2025-11-01) that would be used when a time is selected, instead of defaulting to the current date.

Good news: This feature already exists! The openToDate prop can be used for exactly this purpose.

Example Usage

<DatePicker
  selected={null}
  openToDate={new Date("2025-11-01")}
  onChange={(date) => console.log(date)}
  showTimeSelect
  showTimeSelectOnly
/>

When the user selects a time (e.g., 09:06), the resulting date will be 2025-11-01 09:06:00 instead of {today} 09:06:00.

Tests Added

• should use openToDate as the base date when selecting time with showTimeSelectOnly and selected is null
• should use current date when showTimeSelectOnly is true and neither selected nor openToDate is provided
• should use openToDate for showTimeInput when selected is null

Closes #6032

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.25%. Comparing base (8b66e0d) to head (0398e79).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6157   +/-   ##
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  Coverage   99.25%   99.25%           
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  Files          30       30           
  Lines        3783     3783           
  Branches     1627     1644   +17     
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  Hits         3755     3755           
  Misses         27       27           
  Partials        1        1           

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