fix: serialize local-agent heartbeats

tests/local-agent-serialized-runner.test.mjs

@@ -0,0 +1,39 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import { createSerializedRunner } from "../local-agent/serialized-runner.mjs";
+
+test("createSerializedRunner coalesces overlapping calls into the same promise", async () => {
+  let runs = 0;
+  let release;
+  const gate = new Promise((resolve) => {
+    release = resolve;
+  });
+
+  const runner = createSerializedRunner(async () => {
+    runs += 1;
+    await gate;
+    return { runs };
+  });
+
+  const first = runner();
+  const second = runner();
+  assert.equal(first, second);
+  assert.equal(runs, 1);
+
+  release();
+  const result = await first;
+  assert.deepEqual(result, { runs: 1 });
+});
+
+test("createSerializedRunner allows the next call after the current run finishes", async () => {
+  let runs = 0;
+  const runner = createSerializedRunner(async () => {
+    runs += 1;
+    return runs;
+  });
+
+  assert.equal(await runner(), 1);
+  assert.equal(await runner(), 2);
+  assert.equal(runs, 2);
+});