fix: Fixes dropdown list placement with async options

[pan-kot]

Description

Recalculates dropdown placement in autosuggest, select, and multiselect when options count increase. This ensures the best reasonable placement when options are loaded asynchronously but does not cause unnecessary position change when option list is filtered.

Rel: AWSUI-61474

How has this been tested?

• Manual testing in the new test page
• New integration tests

Before:

Screen.Recording.2026-01-02.at.12.21.40.mov

After:

Screen.Recording.2026-01-02.at.12.22.04.mov

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 97.13%. Comparing base (73fc383) to head (2ff72d8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4147   +/-   ##
=======================================
  Coverage   97.13%   97.13%           
=======================================
  Files         873      873           
  Lines       25562    25568    +6     
  Branches     9239     9242    +3     
=======================================
+ Hits        24830    24836    +6     
  Misses        726      726           
  Partials        6        6           

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