fix(ui5-search): improve arrow navigation with grouping

[dobrinyonkov]

Problem: Arrow keys don't work when navigating through suggestion groups without group headers.

What changed:

• Updated search arrow up/down focus logic to use actual focusable items from the list (flat items, without groups), using the list's listItems getter
• Move getFocusDomRef method to ListItemGroup and return first item if no header is set
• Includes Cypress tests to verify the navigation behavior works as expected.