fix: prevent list item jump on hover in virtual scrolling

[mxschll]

Description

With virtual scrolling enabled, selectable items would "jump" on hover/selection due to border width changes (AWSUI-61303). Virtual scrolling positions items absolutely, so when border width changes from 1px to 2px on hover, the browser recalculates layout dimensions causing visible position shifts.

Additionally, option groups were missing the separating top border.

Before	After

The solution uses box-shadows when virtual scrolling is enabled to simulate thick borders while maintaining thin actual borders. An alternative solution of making all borders the same thickness (hover / non-hover) would change the visual appearance compared to how it's supposed to look.

In summary:

• Eliminates jumping behavior with virtual scrolling enabled
• Maintains visual consistency with non-virtual scrolling
• Adds top separator to option groups

How has this been tested?

Changes ran through screenshot test and have been verified locally.

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 97.17%. Comparing base (11f474d) to head (eb5ce9a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3943   +/-   ##
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  Coverage   97.17%   97.17%           
=======================================
  Files         854      854           
  Lines       24991    24993    +2     
  Branches     8806     8806           
=======================================
+ Hits        24285    24287    +2     
  Misses        658      658           
  Partials       48       48           

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