test(transcription): cover MentionDropdown onSearch callback wiring

For issue #380. Asserts that typing in the search input invokes the
onSearch prop with the current value — characterising the boundary that
PersonMentionEditor relies on for its debounced fetch.

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

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

@@ -7,9 +7,9 @@
  * "no results" / "create new" behaviors. Wiring tests against Tiptap live
  * in PersonMentionEditor.svelte.spec.ts.
  */
-import { describe, it, expect, afterEach } from 'vitest';
+import { describe, it, expect, afterEach, vi } from 'vitest';
 import { cleanup, render } from 'vitest-browser-svelte';
-import { page } from 'vitest/browser';
+import { page, userEvent } from 'vitest/browser';
 import MentionDropdown from './MentionDropdown.svelte';
 import type { components } from '$lib/generated/api';
 
@@ -53,4 +53,20 @@ describe('MentionDropdown — search input', () => {
 		expect(input).not.toBeNull();
 		expect((input as HTMLInputElement).type).toBe('search');
 	});
+
+	it('invokes onSearch with the current value whenever the user types', async () => {
+		const onSearch = vi.fn();
+		render(MentionDropdown, {
+			model: makeModel(),
+			initialQuery: '',
+			onSearch
+		});
+
+		await userEvent.type(page.getByRole('searchbox'), 'Walter');
+
+		await vi.waitFor(() => {
+			expect(onSearch).toHaveBeenCalled();
+			expect(onSearch).toHaveBeenLastCalledWith('Walter');
+		});
+	});
 });