[WC-3418] Combobox accessibility improvements

[HedwigAR]

Pull request type

Bug fix (non-breaking change which fixes an issue)

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Description

Fixes two WCAG accessibility violations in the combobox widget.

1. Decorative icons hidden from screen readers: Added aria-hidden="true" to DownArrow and ClearButton icon wrappers to prevent "image" announcements
2. Empty menu structures removed from accessibility tree: Menu <ul> now only renders when open, preventing screen readers from navigating to empty groups

What should be covered while testing?

Screen Reader Testing:

• Verify decorative icons (arrow, clear button) are NOT announced as "image"
• Verify no empty list structures when menu is closed
• Verify clear button announces with proper label text

Functional Testing:

• Test keyboard navigation (Tab, Enter, Arrow keys, Escape)
• Test single and multi-selection modes
• Verify validation attributes (aria-invalid, aria-describedby) still work