Re-announce "no results" ARIA alert on explicit find navigation (#301135)

* Initial plan

* Re-announce "no results" ARIA alert on explicit find navigation (#301126)

When the user presses Enter/F3 in the find widget to navigate but no
match is found, re-announce "No results found for 'X'" via an ARIA alert.
This ensures screen reader users hear the result even if they pressed
Enter quickly after typing the search term.

Co-authored-by: meganrogge <[email protected]>

* Revert unrelated monaco.d.ts changes

Co-authored-by: meganrogge <[email protected]>

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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: meganrogge <[email protected]>