test(debounce): fix flaky onExit-cancels-debounce test

The test raced a real 150 ms setTimeout: fill('Walter') started the
debounce, then focus + keyboard(Escape) had to complete before 150 ms
elapsed. Under CI load the Playwright CDP round-trips exceeded 150 ms,
letting the debounce fire first.

Fix: install vi.useFakeTimers() after the stable-state setup (so
vi.waitFor()'s real-timer polling still works), freeze the Walter
debounce, let Escape trigger onExit/cancel, then advance fake time
with vi.advanceTimersByTimeAsync() — no real-wall-clock race.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -409,19 +409,24 @@ describe('PersonMentionEditor — onExit cancels pending debounce', () => {
 		await new Promise((r) => setTimeout(r, SEARCH_DEBOUNCE_MS + POST_DEBOUNCE_SLACK_MS));
 		const fetchesBeforeEscape = fetchMock.mock.calls.length;
 
-		// Trigger a new debounced search (queues runSearch after 150 ms), then
-		// immediately Escape *while focus is back in the editor* so Tiptap's
-		// suggestion-plugin Escape handler fires onExit before the debounce.
-		// Without onExit cancelling the pending debounce, runSearch executes
-		// against the now-unmounted dropdown's state.
-		await page.getByRole('searchbox').fill('Walter');
-		// Focus the editor so the Escape lands on Tiptap's suggestion handler.
-		(page.getByRole('textbox').element() as HTMLElement).focus();
-		await userEvent.keyboard('{Escape}');
-
-		// Wait past the debounce window. If onExit did not cancel the pending
-		// debounce, a fetch with q=Walter would still fire here.
-		await new Promise((r) => setTimeout(r, SEARCH_DEBOUNCE_MS + POST_DEBOUNCE_SLACK_MS));
+		// Freeze setTimeout so the 150 ms debounce cannot fire before Escape
+		// triggers onExit. We install fake timers only now — after the setup
+		// above — so that vi.waitFor()'s real-timer polling still worked.
+		vi.useFakeTimers();
+		try {
+			// fill() dispatches the input event synchronously via CDP; by the
+			// time the await resolves, onSearch('Walter') has run and the fake
+			// debounce timer is set.
+			await page.getByRole('searchbox').fill('Walter');
+			// Focus the editor so the Escape lands on Tiptap's suggestion handler.
+			(page.getByRole('textbox').element() as HTMLElement).focus();
+			await userEvent.keyboard('{Escape}');
+			// onExit has now called debouncedSearch.cancel(). Advance past the
+			// debounce window — the cancelled timer must not fire.
+			await vi.advanceTimersByTimeAsync(SEARCH_DEBOUNCE_MS + POST_DEBOUNCE_SLACK_MS);
+		} finally {
+			vi.useRealTimers();
+		}
 
 		const newFetches = fetchMock.mock.calls.slice(fetchesBeforeEscape);
 		const walterFetches = newFetches.filter(