test(document): exercise the real render-failure path in PdfViewer test

The "render failure" test rejected getDocument().promise — the load
path, not the render path — and only asserted a template constant. Now
the fake loads the document successfully and rejects the page render
(the actual #708 wasm-decode failure class), plus a negative companion
asserting the message is absent on a successful render. Also reset
renderTask to null on the render-error path.

Addresses re-review: Felix, Sara (mislabeled test / asserted a constant).

Refs #708

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -7,11 +7,22 @@ import { m } from '$lib/paraglide/messages.js';
 
 afterEach(cleanup);
 
-function makeFailingLibLoader() {
+// Document loads fine, but rendering the page rejects with a non-cancellation
+// error — exactly the wasm-decode failure class from #708. Exercises the real
+// renderCurrentPage path, not the load path.
+function makeRenderFailingLibLoader() {
+	const page = {
+		getViewport: vi.fn().mockReturnValue({ width: 100, height: 100 }),
+		render: vi.fn().mockReturnValue({
+			promise: Promise.reject(new Error('JBig2 failed to initialize')),
+			cancel: vi.fn()
+		}),
+		streamTextContent: vi.fn().mockReturnValue(new ReadableStream())
+	};
 	const lib = {
 		GlobalWorkerOptions: { workerSrc: '' },
 		getDocument: vi.fn().mockReturnValue({
-			promise: Promise.reject(new Error('JBig2 failed to initialize'))
+			promise: Promise.resolve({ numPages: 1, getPage: vi.fn().mockResolvedValue(page) })
 		}),
 		TextLayer: class {
 			render() {
@@ -24,16 +35,27 @@ function makeFailingLibLoader() {
 }
 
 describe('PdfViewer — render failure', () => {
-	it('shows the localized failure message and a download link, not a blank canvas', async () => {
+	it('shows the localized failure message and a download link when the page render rejects', async () => {
 		render(PdfViewer, {
 			url: '/api/documents/test/file',
 			documentId: 'test',
-			libLoader: makeFailingLibLoader()
+			libLoader: makeRenderFailingLibLoader()
 		});
 
 		await expect.element(page.getByText(m.doc_render_failed())).toBeVisible();
 		await expect.element(page.getByRole('link', { name: m.doc_download_link() })).toBeVisible();
 	});
+
+	it('does not show the failure message when the page renders successfully', async () => {
+		render(PdfViewer, {
+			url: '/api/documents/test/file',
+			documentId: 'test',
+			libLoader: makeFakeLibLoader()
+		});
+
+		await expect.element(page.getByRole('button', { name: 'Zurück' })).toBeVisible();
+		expect(document.body.textContent).not.toContain(m.doc_render_failed());
+	});
 });
 
 describe('PdfViewer — empty / error states', () => {

frontend/src/lib/document/viewer/usePdfRenderer.svelte.ts

@@ -111,6 +111,7 @@ export function createPdfRenderer(libLoader: LibLoader = defaultLibLoader) {
 			// A real decode/render failure (e.g. a wasm decoder that could not
 			// initialise) — surface a localized message instead of leaving a
 			// silent blank canvas. Never leak the raw pdf.js error text.
+			renderTask = null;
 			error = m.doc_render_failed();
 			return;
 		}