Fix command palette first item lookup under Vue 3

What does this MR do and why?

Fixes a console error in the command palette (super sidebar global search) that appears when GitLab runs on the Vue 3 compat build:

Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'toggle')
  at Proxy.highlightFirstCommand (global_search.vue:407)

highlightFirstCommand() and runFirstCommand() used getFocusableOptions()[0]?.firstChild and assumed the first child node of the .gl-new-dropdown-item list item is the clickable element. Under Vue 3 / @vue/compat, the rendered dropdown item can start with a fragment anchor node — an empty text node or a <!--v-if--> comment. Those nodes have no classList or click(), so highlighting the first command throws, and pressing Enter to run the first command breaks.

References

• Review thread where the bug was reported: !250742 (comment 3712917080)

Screenshots or screen recordings

How to set up and validate locally

1. Run GDK with the Vue 3 build (VUE_VERSION=3 for webpack).
2. Open global search and type ~ inside a project to enter file search.
3. Confirm there is no console error and the first result is highlighted.
4. Press Enter and confirm the first file opens.