feat: Add disabled filter option to test utils to button-dropdown

[Who-is-PS]

Description

Add { disabled?: boolean } filter option to findItems() and findItemById() test utils for ButtonDropdown. Follows the same pattern as TreeView's findItems({ expanded?: boolean }).

Related links, issue #, if available: AWSUI-61729

How has this been tested?

Unit tests added in src/button-dropdown/tests/button-dropdown-items.test.tsx

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 97.19%. Comparing base (40696d3) to head (e8fea4a).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4244   +/-   ##
=======================================
  Coverage   97.19%   97.19%           
=======================================
  Files         886      886           
  Lines       26017    26029   +12     
  Branches     9431     9438    +7     
=======================================
+ Hits        25287    25299   +12     
  Misses        683      683           
  Partials       47       47           

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