fix: acknowledge pi maintenance barrier

backend/test/maintenance-gate.test.ts

@@ -0,0 +1,19 @@
+import { test, expect } from "vitest";
+import { MaintenanceBarrier } from "../src/runtime/maintenance-gate.js";
+
+test("activation waits for an in-flight admission lease and rejects later admissions", async () => {
+  const gate = new MaintenanceBarrier();
+  const release = gate.acquire();
+  expect(release).toBeTypeOf("function");
+  let acknowledged = false;
+  const activation = gate.activate().then(() => { acknowledged = true; });
+  await Promise.resolve();
+  expect(acknowledged).toBe(false);
+  expect(gate.acquire()).toBeUndefined();
+  release?.();
+  await activation;
+  expect(acknowledged).toBe(true);
+  expect(gate.status()).toEqual({ active: true, admissions: 0 });
+  gate.deactivate();
+  expect(gate.acquire()).toBeTypeOf("function");
+});