test(discussion): atomically clear mention searchbox to kill CI flake

userEvent.clear deletes per-keystroke, so intermediate values 'Au'/'A'
transit through the bound searchQuery and each schedules a debounced
fetch. When CI keystroke jitter exceeds SEARCH_DEBOUNCE_MS (150 ms), an
intermediate timer fires before the input reaches '' and the count
assertion sees a phantom q=Au call. fill('') drops a single input event
so the empty-query branch wins deterministically — same pattern this
test file already uses for fill('Walter').

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts

@@ -283,7 +283,15 @@ describe('PersonMentionEditor — AC-2/3: search input drives fetch', () => {
 
 		const fetchesBeforeClear = fetchMock.mock.calls.length;
 
-		await userEvent.clear(page.getByRole('searchbox'));
+		// `fill('')` atomically sets the input value in a single input event —
+		// same rationale as the `fill('Walter')` call above. `userEvent.clear`
+		// deletes per-keystroke, so intermediate values 'Au'/'A' transit through
+		// the bound `searchQuery` and each schedules a debounced fetch. Under
+		// CI-runner jitter, if two consecutive keystrokes land more than
+		// SEARCH_DEBOUNCE_MS (150 ms) apart, an intermediate timer fires before
+		// the input reaches '' and adds a phantom call — unrelated to the
+		// contract under test, which is "empty query => no fetch".
+		await page.getByRole('searchbox').fill('');
 
 		// Negative assertion: wait past the debounce window to confirm no
 		// trailing fetch was scheduled. Removing this wait would mask a