fix: wrap scrollToIndex in requestAnimationFrame

[mxschll]

Description

Fixes incorrect scroll positioning in virtual lists by deferring scrollToIndex calls with requestAnimationFrame. This ensures virtual items are measured before scrolling occurs.

Related links, issue #, if available: AWSUI-61506

How has this been tested?

• Verified changes manually
• Changes wend through pipeline
• Dry-run build request (ID: 7659319803)

Visual changes in scroll positions are expected, since it scrolls correctly.

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 97.14%. Comparing base (1a5715e) to head (831d66c).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4134   +/-   ##
=======================================
  Coverage   97.14%   97.14%           
=======================================
  Files         873      873           
  Lines       25642    25648    +6     
  Branches     9284     9281    -3     
=======================================
+ Hits        24910    24916    +6     
+ Misses        726      685   -41     
- Partials        6       47   +41     

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