fix: add safety timeout to session.compact() to prevent lane deadlock (#16533)

Merged via /review-pr -> /prepare-pr -> /merge-pr.

Prepared head SHA: 21e4045add
Co-authored-by: BinHPdev <219093083+BinHPdev@users.noreply.github.com>
Co-authored-by: gumadeiras <5599352+gumadeiras@users.noreply.github.com>
Reviewed-by: @gumadeiras

src/agents/pi-embedded-runner.compaction-safety-timeout.test.ts

@@ -0,0 +1,45 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+import {
+  compactWithSafetyTimeout,
+  EMBEDDED_COMPACTION_TIMEOUT_MS,
+} from "./pi-embedded-runner/compaction-safety-timeout.js";
+
+describe("compactWithSafetyTimeout", () => {
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  it("rejects with timeout when compaction never settles", async () => {
+    vi.useFakeTimers();
+    const compactPromise = compactWithSafetyTimeout(() => new Promise<never>(() => {}));
+    const timeoutAssertion = expect(compactPromise).rejects.toThrow("Compaction timed out");
+
+    await vi.advanceTimersByTimeAsync(EMBEDDED_COMPACTION_TIMEOUT_MS);
+    await timeoutAssertion;
+    expect(vi.getTimerCount()).toBe(0);
+  });
+
+  it("returns result and clears timer when compaction settles first", async () => {
+    vi.useFakeTimers();
+    const compactPromise = compactWithSafetyTimeout(
+      () => new Promise<string>((resolve) => setTimeout(() => resolve("ok"), 10)),
+      30,
+    );
+
+    await vi.advanceTimersByTimeAsync(10);
+    await expect(compactPromise).resolves.toBe("ok");
+    expect(vi.getTimerCount()).toBe(0);
+  });
+
+  it("preserves compaction errors and clears timer", async () => {
+    vi.useFakeTimers();
+    const error = new Error("provider exploded");
+
+    await expect(
+      compactWithSafetyTimeout(async () => {
+        throw error;
+      }, 30),
+    ).rejects.toBe(error);
+    expect(vi.getTimerCount()).toBe(0);
+  });
+});