Fix: Scrollable dropdown does not highlight current selection (#4958)

[solomona2003]

Description

Fixes #4958

This PR fixes two issues with the scrollable dropdown component:

1. Selection highlighting: The currently selected item is now properly highlighted when the dropdown is reopened
2. Dynamic loading: Dropdown items are loaded based on actual vocabulary size

Changes Made

Selection Highlighting

• Highlight current value when menu opens (setSelectedIndexToCurrentValue)
• Treat 'Clear selection' as index 0, adjust item indices (i+1)
• Arrow key navigation respects clear item (totalItems)
• Enter selects correct item or clears when index = 0
• Skip disabled items in scrollToSelected
• Selected items now highlighted correctly up to position 200

Dynamic Loading

• Uses actual vocabulary size from pageInfo.totalElements
• Caps at 200 items to prevent loading huge vocabularies
• Performance scaled: 50 items (Good UX), 100 items (Acceptable), 200 items (Borderline)

Testing

Tested with submission forms using Type and Language dropdowns.

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[lgeggleston]

Hi! Tested this fix via the Type and Language dropdowns, looks good and works as expected. Nice catch on the keyboard navigation to 'Clear selection' too!

Also adding backport tags to 8.x and 7.x; after comparing code it looks like this fix should be able to auto backport smoothly.

[tdonohue]

👍 Thanks @solomona2003 ! The code looks good to me, and glad to see that @lgeggleston tested it thoroughly

[dspace-bot]

Backport failed for dspace-7_x, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin dspace-7_x
git worktree add -d .worktree/backport-5020-to-dspace-7_x origin/dspace-7_x
cd .worktree/backport-5020-to-dspace-7_x
git switch --create backport-5020-to-dspace-7_x
git cherry-pick -x 44a3fcd7d5808d61caa8af5432487841cb8490b1

[dspace-bot]

Backport failed for dspace-8_x, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin dspace-8_x
git worktree add -d .worktree/backport-5020-to-dspace-8_x origin/dspace-8_x
cd .worktree/backport-5020-to-dspace-8_x
git switch --create backport-5020-to-dspace-8_x
git cherry-pick -x 44a3fcd7d5808d61caa8af5432487841cb8490b1

[dspace-bot]

Successfully created backport PR for dspace-9_x:

• [Port dspace-9_x] Fix: Scrollable dropdown does not highlight current selection (#4958) #5086

[tdonohue]

@solomona2003 : We were able to auto-backport this to 9.x. But, the backports to 8.x and 7.x failed. This will need to wait on a volunteer to backport it further. Please let me know if you are interested in backporting this to our dspace-8_x or dspace-7_x branches.

Thanks again!