fix: restore existsSync in node:fs test mock

src/lib/services/import/core/download-errors.test.ts

@@ -2,7 +2,13 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 
 const write = vi.hoisted(() => vi.fn());
-vi.mock("node:fs", () => ({ promises: { writeFile: write } }));
+vi.mock("node:fs", async (importOriginal) => {
+	const actual = await importOriginal();
+	return {
+		...actual,
+		existsSync: vi.fn().mockReturnValue(false),
+	};
+});
 
 import { downloadFile, tryDownloadCandidates } from "./download";