lint: fix biome issues in new test files and helpers

src/lib/services/translation-pool.test.ts

@@ -10,7 +10,8 @@ vi.mock("node:worker_threads", () => {
 	class FakeWorker {
 		private handlers: Record<string, Handler[]> = {};
 		on(event: string, handler: Handler) {
-			(this.handlers[event] ??= []).push(handler);
+			if (!this.handlers[event]) this.handlers[event] = [];
+			this.handlers[event].push(handler);
 			return this;
 		}
 		emit(event: string, ...args: unknown[]) {