fix: Adding default aria label for open calendar button

[Harsh-Anand-Singh]

Description

This PR adds a default aria-label for the DatePicker calendar button, fixing an accessibility issue where the button had no label when openCalendarAriaLabel was not provided.

Related links, issue #, if available: AWSUI-61446

How has this been tested?

Added unit tests to verify the calendar button has the correct default aria-label with and without a selected date.

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]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.99%. Comparing base (9edb184) to head (92df6e1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4043   +/-   ##
=======================================
  Coverage   96.99%   96.99%           
=======================================
  Files         861      861           
  Lines       25210    25212    +2     
  Branches     9097     9099    +2     
=======================================
+ Hits        24452    24454    +2     
+ Misses        752      711   -41     
- Partials        6       47   +41     

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