fix(FN-1068): switch vitest pool from vmThreads to threads to fix node:sqlite error

- Change vitest pool from vmThreads to threads in engine package
- Resolves node:sqlite compatibility issue caused by vmThreads isolation
- vmThreads creates separate VM contexts incompatible with native SQLite module

packages/engine/vitest.config.ts

@@ -7,7 +7,7 @@ export default defineConfig({
     include: ["src/**/*.test.ts"],
     maxWorkers,
     fileParallelism: true,
-    pool: "vmThreads",
+    pool: "threads",
     coverage: {
       enabled: false,
       reporter: ["text", "html", "json"],