Fix search arrow key focus handoff

[tracygardner]

Motivation

• The toolbox search input could lose its keydown handler when Blockly mutates the DOM, causing ArrowDown/Tab to keep focus in the search box instead of moving into the flyout.

Description

• Add a guard using the search.dataset.searchFlyoutWired flag to avoid attaching duplicate keydown handlers to the toolbox search input and call wireSearchInput() from the mutation observer so newly created search inputs are wired.

Testing

• No automated tests were executed for this change.

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Codex Task