fix: Patch Screan reader issue in multiselect selected items

[NathanZlion]

Description

Fixed MultiSelect component accessibility issue where screen readers incorrectly announced "not-selected" for all options, including those that were actually selected. This prevented users relying on assistive technology from distinguishing between selected and unselected options.

• Added aria-multiselectable="true"__ to the listbox element when in multiselect mode
• Added both aria-selected attributes to multiselect options
• Retained selectedAriaLabel in live region announcements AWSUI-61639

Related links, issue #, if available: n/a
AWSUI-61639

How has this been tested?

• Tested with VoiceOver (macOS) - announcements work correctly, no more "not selected" false announcements
• Done tests for MAC as well with the help of A11Y team, that's how we decided to keep the aria-live region selectedAriaLabel because NVDA and early version of JAWS (e.g. 2022, which is still pretty commonly used) have a bug around announcing the selected state.

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

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.16%. Comparing base (0f8232f) to head (10e89c8).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
src/select/parts/virtual-list.tsx	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4199   +/-   ##
=======================================
  Coverage   97.15%   97.16%           
=======================================
  Files         878      879    +1     
  Lines       25716    25750   +34     
  Branches     9297     9305    +8     
=======================================
+ Hits        24985    25019   +34     
+ Misses        725      684   -41     
- Partials        6       47   +41     

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.19%. Comparing base (825e839) to head (401a6c5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4199   +/-   ##
=======================================
  Coverage   97.19%   97.19%           
=======================================
  Files         886      886           
  Lines       25996    25998    +2     
  Branches     9416     9418    +2     
=======================================
+ Hits        25267    25269    +2     
  Misses        682      682           
  Partials       47       47           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.