test(FN-4900): add failing recursion regression

Fusion-Task-Id: FN-4900
Fusion-Task-Lineage: ec7d26d7-4d38-4afd-b029-4b68d5f2db6f

packages/engine/src/__tests__/pi-prompt-with-fallback-recursion.test.ts

@@ -0,0 +1,17 @@
+import { describe, expect, it, vi } from "vitest";
+import { DefaultPiRuntime } from "../runtime-resolution.js";
+
+describe("pi promptWithFallback recursion guard (FN-4900)", () => {
+  it("does not recurse when a bare session is wrapped by DefaultPiRuntime", async () => {
+    const runtime = new DefaultPiRuntime();
+    const prompt = vi.fn(async () => undefined);
+    const session = { prompt } as any;
+
+    session.promptWithFallback = (input: string, options?: unknown) =>
+      runtime.promptWithFallback(session, input, options);
+
+    await expect(session.promptWithFallback("hello")).resolves.toBeUndefined();
+    expect(prompt).toHaveBeenCalledTimes(1);
+    expect(prompt).toHaveBeenCalledWith("hello", undefined);
+  });
+});