feat: add internationalization support for openCalendarAriaLabel

[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 97.12%. Comparing base (c02fc0d) to head (44b0751).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4082   +/-   ##
=======================================
  Coverage   97.12%   97.12%           
=======================================
  Files         864      864           
  Lines       25325    25328    +3     
  Branches     9137     9138    +1     
=======================================
+ Hits        24597    24600    +3     
  Misses        722      722           
  Partials        6        6           

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