Prevent removal of input that is the parent dropdown element

What does this MR do?

• Prevents GitLab Dropdowns from removing its attached element if it is an input
• Fix the navigation search bar to work after meta-clicking a dropdown item (opening the link in a new tab)
• Replace search input with the selected option's text value if present

Why was this MR needed?

• When meta-clicking a link in the dropdown, the GitLab Dropdown would remove the search input element if it didn't have anything in it.
• When the input element was removed, there wasn't a way to search again unless the page was reloaded.

Screenshots (if relevant)

• Replaces search content with selected item text
• Opens selected link in new tab when meta-clicked, and dropdown search continues to operate normally

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #25312 (closed)