fix: a11y fix in multiselect

[2nikhiltom]

Closes #20890

Add aria-checked to MultiSelect and FilterableMultiSelect components to announce checkbox state
screen readers weren't announcing checkbox state (checked/unchecked) when navigating MultiSelect options with arrow keys.

Changelog

New

Added aria-checked attribute

Testing / Reviewing

1/Open MultiSelect dropdown
2. Navigate options with arrow keys
3. Verify screen reader announces "checked" or "not checked" state

PR Checklist

As the author of this PR, before marking ready for review, confirm you:

• Reviewed every line of the diff
• Updated documentation and storybook examples
• Wrote passing tests that cover this change
• Addressed any impact on accessibility (a11y)
• Tested for cross-browser consistency
• Validated that this code is ready for review and status checks should pass

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.10%. Comparing base (ea2b873) to head (bc9615f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21177      +/-   ##
==========================================
- Coverage   85.14%   85.10%   -0.05%     
==========================================
  Files         530      530              
  Lines       40900    40902       +2     
  Branches     6346     6345       -1     
==========================================
- Hits        34826    34808      -18     
- Misses       5913     5933      +20     
  Partials      161      161              

Flag	Coverage Δ
main-packages	85.73% <100.00%> (+<0.01%)	⬆️
web-components	84.74% <ø> (-0.08%)	⬇️

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[riddhybansal]

LGTM 💯