Fix combobox search items that show above the sticky search

[ump45nose]

Description

The sticky combobox search used top: 0. The menu has block padding. The search stuck below that padding. Scrolled options showed in that strip.

The search now uses top: calc(-1 * var(--menu-padding-y, .25rem)). It covers the menu block padding when the menu scrolls. The unscrolled layout stays the same.

npm run css-lint passed. npm run css passed. npm run lint passed. No combobox Sass test exists under scss/tests. A headless Chrome page confirmed the bug on main, then confirmed the fix. The hit target in the padding strip changed from a menu item to .combobox-search. The unscrolled menu pixels did not change.

This change was prepared with AI assistance (Cursor agent) and reviewed before submission.

Motivation & Context

A combobox menu with a search field lets option text show above the search while the user scrolls. The sticky search did not cover the menu block padding.

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Related issues

Fixes #43036