test(transcription): polish @mention test docstrings and tighten clip assert

Round-4 polish from Sara (#11199) and Felix (#11186):
- Replace setTimeout(50) in stale-response race with tick() — matches
  round-3 pattern Sara verified in the sticky-takeover test.
- Add intent comment above the "clear input" wait — it is a negative
  assertion that must not be optimised away.
- Tighten displayName-clip assert from <=100 to ===100 so the test
  discriminates "clip works" from "clip works AND nothing weakened it".
- JSDoc POST_DEBOUNCE_SLACK_MS with the calibration rationale.

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

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

@@ -8,6 +8,7 @@
 import { describe, it, expect, vi, afterEach } from 'vitest';
 import { cleanup, render } from 'vitest-browser-svelte';
 import { page, userEvent } from 'vitest/browser';
+import { tick } from 'svelte';
 import PersonMentionEditorHost from './PersonMentionEditor.test-fixture.svelte';
 import type { components } from '$lib/generated/api';
 import { m } from '$lib/paraglide/messages.js';
@@ -18,7 +19,12 @@ import { SEARCH_DEBOUNCE_MS } from './mentionConstants';
 type Person = components['schemas']['Person'];
 type PersonMention = components['schemas']['PersonMention'];
 
-// Slack on top of the debounce to absorb CI jitter (total 500 ms is generous).
+/**
+ * Headroom above SEARCH_DEBOUNCE_MS for the debounce-window wait
+ * assertions in this file. 350 ms is calibrated against CI-runner jitter
+ * we observed pre-#629; dropping it below ~200 ms reintroduces flake.
+ * See PR #629 round-2 review comment #10935 (Sara).
+ */
 const POST_DEBOUNCE_SLACK_MS = 350;
 
 const AUGUSTE: Person = {
@@ -275,6 +281,9 @@ describe('PersonMentionEditor — AC-2/3: search input drives fetch', () => {
 
 		await userEvent.clear(page.getByRole('searchbox'));
 
+		// Negative assertion: wait past the debounce window to confirm no
+		// trailing fetch was scheduled. Removing this wait would mask a
+		// re-introduction of the keystroke-driven items() fetch.
 		await new Promise((r) => setTimeout(r, SEARCH_DEBOUNCE_MS + POST_DEBOUNCE_SLACK_MS));
 
 		expect(fetchMock.mock.calls.length).toBe(fetchesBeforeClear);
@@ -332,7 +341,10 @@ describe('PersonMentionEditor — stale-response race', () => {
 
 		// The stale fetch now resolves with persons. The dropdown must stay empty.
 		resolveFetch({ ok: true, json: () => Promise.resolve([AUGUSTE]) });
-		await new Promise((r) => setTimeout(r, 50));
+		// Flush pending Svelte reactivity so any (non-)update from the stale
+		// fetch resolution has landed before we assert. expect.element already
+		// polls, so no fixed-timeout fallback is needed. Sara on PR #629 round 4.
+		await tick();
 
 		await expect.element(page.getByText('Auguste Raddatz')).not.toBeInTheDocument();
 	});
@@ -528,7 +540,10 @@ describe('PersonMentionEditor — AC-1: typed text as displayName', () => {
 
 		await vi.waitFor(() => {
 			expect(host.snapshot.mentionedPersons).toHaveLength(1);
-			expect(host.snapshot.mentionedPersons[0].displayName.length).toBeLessThanOrEqual(100);
+			// Tight assertion: input is 105 chars, cap is exactly 100. Using
+			// `toHaveLength(100)` discriminates "clip works" from "clip works
+			// AND nothing weakened it to e.g. 95". Sara on PR #629 round 4.
+			expect(host.snapshot.mentionedPersons[0].displayName).toHaveLength(100);
 		});
 	});