refactor(pdf-viewer-tests): extract shared fake, add loadDocument error path, fix assertions

- Extract makeFakePdfjsLib / makeFakeLibLoader to testHelpers.ts — single
  source of truth used by both PdfViewer.svelte.test.ts and
  usePdfRenderer.svelte.test.ts; removes the diverging-fidelity DRY violation
  flagged by @felixbrandt and @saraholt in the PR review
- Add 'loadDocument sets error and loading=false when getDocument().promise
  rejects' test to usePdfRenderer.svelte.test.ts — closes the error-path gap
  flagged by @felixbrandt and @saraholt
- Replace toBeInTheDocument() with toBeVisible() in the three absorbed
  spec-file tests — uniform assertion style across the loaded-state describe
  block, as flagged by @felixbrandt

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

frontend/src/lib/document/viewer/PdfViewer.svelte.test.ts

@@ -1,40 +1,11 @@
 import { describe, it, expect, vi, afterEach } from 'vitest';
 import { cleanup, render } from 'vitest-browser-svelte';
 import { page } from 'vitest/browser';
-import type { LibLoader } from '$lib/document/viewer/usePdfRenderer.svelte';
 import PdfViewer from './PdfViewer.svelte';
+import { makeFakeLibLoader } from './testHelpers';
 
 afterEach(cleanup);
 
-function makeFakePdfjsLib() {
-	class TextLayerMock {
-		render() {
-			return Promise.resolve();
-		}
-		cancel() {}
-	}
-
-	return {
-		GlobalWorkerOptions: { workerSrc: '' },
-		getDocument: vi.fn().mockReturnValue({
-			promise: Promise.resolve({
-				numPages: 2,
-				getPage: vi.fn().mockResolvedValue({
-					getViewport: vi.fn().mockReturnValue({ width: 595, height: 842 }),
-					render: vi.fn().mockReturnValue({ promise: Promise.resolve() }),
-					streamTextContent: vi.fn().mockReturnValue(new ReadableStream())
-				})
-			})
-		}),
-		TextLayer: TextLayerMock
-	} as unknown as typeof import('pdfjs-dist');
-}
-
-function makeFakeLibLoader(): LibLoader {
-	const fakePdfjs = makeFakePdfjsLib();
-	return vi.fn().mockResolvedValue([fakePdfjs, { default: '' }] as const);
-}
-
 describe('PdfViewer — empty / error states', () => {
 	it('renders the no-file placeholder when url is empty', async () => {
 		render(PdfViewer, { url: '', libLoader: makeFakeLibLoader() });
@@ -293,18 +264,18 @@ describe('PdfViewer — loaded state', () => {
 
 	it('shows previous and next page navigation buttons', async () => {
 		render(PdfViewer, { url: '/api/documents/test-id/file', libLoader: makeFakeLibLoader() });
-		await expect.element(page.getByRole('button', { name: /zurück/i })).toBeInTheDocument();
-		await expect.element(page.getByRole('button', { name: /weiter/i })).toBeInTheDocument();
+		await expect.element(page.getByRole('button', { name: /zurück/i })).toBeVisible();
+		await expect.element(page.getByRole('button', { name: /weiter/i })).toBeVisible();
 	});
 
 	it('shows zoom controls', async () => {
 		render(PdfViewer, { url: '/api/documents/test-id/file', libLoader: makeFakeLibLoader() });
-		await expect.element(page.getByRole('button', { name: /vergrößern/i })).toBeInTheDocument();
-		await expect.element(page.getByRole('button', { name: /verkleinern/i })).toBeInTheDocument();
+		await expect.element(page.getByRole('button', { name: /vergrößern/i })).toBeVisible();
+		await expect.element(page.getByRole('button', { name: /verkleinern/i })).toBeVisible();
 	});
 
 	it('displays the page counter once the PDF has loaded', async () => {
 		render(PdfViewer, { url: '/api/documents/test-id/file', libLoader: makeFakeLibLoader() });
-		await expect.element(page.getByText(/1\s*\/\s*2/)).toBeInTheDocument();
+		await expect.element(page.getByText(/1\s*\/\s*2/)).toBeVisible();
 	});
 });