Add ArrowUp/ArrowDown keyboard navigation from toolbox search input

[tracygardner]

Motivation

• Improve keyboard accessibility by allowing users to press ArrowDown/ArrowUp while focused in the toolbox search input to move focus into the toolbox item list.
• Ensure that DOM focus and the toolbox selection highlight remain synchronized when navigation moves away from the search input.

Description

• Add a keydown listener to the search input in createDom_() of node_modules/@blockly/toolbox-search/src/toolbox_search.ts that handles ArrowDown and ArrowUp keys.
• When an arrow key is pressed the handler calls event.preventDefault(), blurs the search input via getFocusableElement().blur(), computes the adjacent toolbox item using this.parentToolbox_.getToolboxItems() and the current index, and then moves selection and focus to that item by calling this.parentToolbox_.setSelectedItem(nextItem) (if selectable) and Blockly.getFocusManager().focusNode(nextItem).
• The existing Escape behavior and existing keyup search/match logic are preserved.
• All changes are contained in toolbox_search.ts and use the focus APIs already exposed by Blockly (getFocusableElement() and getFocusManager().focusNode).

Testing

• No automated tests were run for this change.

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Codex Task