refactor(test): dedupe pi embedded subscribe e2e harness

src/agents/pi-embedded-subscribe.e2e-harness.ts

@@ -0,0 +1,18 @@
+import type { subscribeEmbeddedPiSession } from "./pi-embedded-subscribe.js";
+
+type PiSession = Parameters<subscribeEmbeddedPiSession>[0]["session"];
+
+export function createStubSessionHarness(): {
+  session: PiSession;
+  emit: (evt: unknown) => void;
+} {
+  let handler: ((evt: unknown) => void) | undefined;
+  const session = {
+    subscribe: (fn: (evt: unknown) => void) => {
+      handler = fn;
+      return () => {};
+    },
+  } as unknown as PiSession;
+
+  return { session, emit: (evt: unknown) => handler?.(evt) };
+}