Replaces 16 setTimeout(350ms / 30ms / 50ms) sleeps with vi.waitFor on
the actual signal — popup listbox appearance/disappearance, option
aria-selected state — so the test no longer races the 200ms internal
debounce against the real clock under CI load.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Textarea props (placeholder, rows, disabled), popup not shown
initially, popup opens on @ + query, empty results from API,
HTTP error → empty popup, Enter submits when popup closed,
Shift+Enter does not submit, Escape closes popup, Arrow{Up,Down}
navigation, Enter with no results.
12 tests covering ~30 branches in MentionEditor.
Refs #496.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>