Expand toolbox subcategories on keyboard focus

[tracygardner]

Motivation

• Improve keyboard accessibility so collapsible toolbox categories with nested subcategories expand when they receive keyboard focus, matching mouse click behavior.
• Address cases where keyboard users could focus a category but not see or open its flyout contents.

Description

• Added ensureKeyboardFocusedSelection_() to CustomCollapsibleToolboxCategory to set selection, expand the category, and show flyout contents without forcing pointer-specific focus handling in toolbox.js.
• Wired a focusin handler on the category rowDiv_ to call ensureKeyboardFocusedSelection_() when the toolbox has keyboard focus so keyboard navigation expands subcategories.
• Preserved existing pointer behavior (ensurePointerFocusedSelection_()), keeping mouse interactions unchanged.

Testing

• Ran npm run build which completed successfully without errors.
• Started the dev server and executed a Playwright script that tabs into the toolbox and captures a screenshot to confirm categories expand on keyboard focus, producing an artifact image for manual inspection.

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Codex Task