fix: Only provide date strings to isValidRange when date range picker is set to dateOnly

[avinashbot]

Description

Self-explanatory - but the bigger question is whether it'll break anyone. Sending it through my pipeline to check.

Related links, issue #, if available: AWSUI-18012

How has this been tested?

Added a unit test.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (1a41997) 94.70% compared to head (f996594) 94.58%.

Files	Patch %	Lines
src/date-range-picker/dropdown.tsx	0.00%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1516      +/-   ##
==========================================
- Coverage   94.70%   94.58%   -0.13%     
==========================================
  Files         654      654              
  Lines       17790    17792       +2     
  Branches     5866     5866              
==========================================
- Hits        16848    16828      -20     
- Misses        877      899      +22     
  Partials       65       65              

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