test(planner): clarify server.test.ts error-branch test name

"when backend returns error" → "when data is undefined (error response
without data)" — documents that the guard is data?.suggestions ?? [],
not error field inspection.

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

frontend/src/routes/(app)/planner/server.test.ts

@@ -59,7 +59,7 @@ describe('GET /planner — suggestions route handler', () => {
 		expect(body.suggestions[1].recipe.name).toBe('Nudeln');
 	});
 
-	it('returns { suggestions: [] } when backend returns error', async () => {
+	it('returns { suggestions: [] } when data is undefined (error response without data)', async () => {
 		mockGet.mockResolvedValueOnce({ data: undefined, error: { status: 500 } });
 
 		const url = new URL(`http://localhost/planner?planId=${PLAN_UUID}&date=${DATE}`);