fix: issue 20774 Border on selected list item on dropdown

[sbriones97]

Closes #20774

fix: issue #20774 -> Border on selected list item on dropdown

Changelog

New

• Css property for active selected item on list

Changed

• border start 0

Removed

• N/A

Testing / Reviewing

Open Dropdown and select an item

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
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[riddhybansal]

Hey thanks for contributing!! Your contribution is appreciated.
This looks good!
One minor suggestion: consider using border-block-start: none instead of border-block-start: 0 to stay consistent with other border declarations in the Carbon codebase.

[riddhybansal]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.46%. Comparing base (84b09fc) to head (b3a52ed).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #20779   +/-   ##
=======================================
  Coverage   92.46%   92.46%           
=======================================
  Files         508      508           
  Lines       35753    35753           
  Branches     5673     5673           
=======================================
  Hits        33058    33058           
  Misses       2550     2550           
  Partials      145      145           

Flag	Coverage Δ
main-packages	85.49% <ø> (ø)
web-components	97.19% <ø> (ø)

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[riddhybansal]

Question for @thyhmdo:
I notice different focus behaviors between Dropdown and ComboBox:

In ComboBox, focus remains fixed on the selected item when opened
In Dropdown, focus moves with the mouse cursor and loses focus from options if the cursor isn't over any option (only hover state changes)

Are these behaviors intentionally different, or should they be consistent? The documentation doesn't seem to clarify which approach is correct according to design system guidelines. Could you provide guidance?

[sbriones97]

I have read the DCO document and I hereby sign the DCO.

[sbriones97]

recheck

[sbriones97]

@riddhybansal Was my last commit ok to continue?