test(frontend): unit tests exercise named SSE events (fidelity) + cleanup

FakeEventSource.addEventListener was a no-op, so emit() only drove onmessage.
Production relies on addEventListener for the backend's NAMED events
(event: ui_request), so the unit tests could pass while prod silently broke.

- fakeEventSource: store named handlers in a Map<string,Set>; add emitNamed()
  that dispatches to them; keep emit() for the unnamed/default onmessage path
- useSessionStream.test: ui_request now driven via emitNamed (production path);
  add a separate test for the unnamed text_delta path via plain emit
- f1-loop.test: widget emission switched to emitNamed("ui_request", ...)
- verified: tests FAIL if addEventListener wiring is removed from
  useSessionStream (then restored)
- cleanup: fake-pi.mjs drops unused execFileSync import, uses static spawnSync

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/e2e/fixtures/fake-pi.mjs

@@ -4,15 +4,14 @@
 // exec()s the scripted fake-pi-rpc.mjs with the F1 disambiguation scenario.
 import { fileURLToPath } from "node:url";
 import { dirname, resolve } from "node:path";
-import { execFileSync } from "node:child_process";
+import { spawnSync } from "node:child_process";
 
 const __dir = dirname(fileURLToPath(import.meta.url));
 const fakePiRpc = resolve(__dir, "../../..", "harness/tests/fake_pi/fake_pi_rpc.mjs");
 const scenario = resolve(__dir, "../../..", "harness/tests/fake_pi/scripts/f1_disambiguation.json");
 
-// exec replaces the current process so the backend's child handle stays valid.
-// On Node.js we use spawnSync with stdio inherit to approximate exec().
-const { spawnSync } = await import("node:child_process");
+// Run the scripted fake-pi-rpc on this process's stdio so the backend's child
+// handle (stdin/stdout) drives the JSONL protocol directly.
 const result = spawnSync(process.execPath, [fakePiRpc, scenario], {
   stdio: "inherit",
   env: { ...process.env },

frontend/src/shell/f1-loop.test.tsx

@@ -41,9 +41,10 @@ test("F1: create session -> widget via SSE -> respond -> POST /response", async
   // Wait for the session to be created and SSE to connect
   await waitFor(() => expect(FakeEventSource.instances).toHaveLength(1));
 
-  // Simulate the backend emitting the F1 widget
+  // Simulate the backend emitting the F1 widget via a NAMED SSE event
+  // (`event: ui_request`) — the same addEventListener path production uses.
   act(() =>
-    FakeEventSource.instances[0].emit({
+    FakeEventSource.instances[0].emitNamed("ui_request", {
       type: "ui_request",
       ui_request: {
         id: "u1",

frontend/src/stream/useSessionStream.test.tsx

@@ -10,16 +10,25 @@ beforeEach(() => {
   useSessionStore.getState().resetSession();
 });
 
-test("opens an EventSource for the session and feeds events to the store", () => {
+test("opens an EventSource and feeds NAMED events to the store", () => {
   renderHook(() => useSessionStream("s1"));
   const es = FakeEventSource.instances[0];
   expect(es.url).toContain("/sessions/s1/events");
+  // Backend sends `event: ui_request` (named) — drive the addEventListener path
+  // that production relies on, not the unnamed onmessage fallback.
   act(() =>
-    es.emit({ type: "ui_request", ui_request: { id: "u1", widget: "select" } })
+    es.emitNamed("ui_request", { type: "ui_request", ui_request: { id: "u1", widget: "select" } })
   );
   expect(useSessionStore.getState().pendingWidget?.id).toBe("u1");
 });
 
+test("also feeds UNNAMED (default message) events to the store", () => {
+  renderHook(() => useSessionStream("s1"));
+  const es = FakeEventSource.instances[0];
+  act(() => es.emit({ type: "text_delta", text: "hello" }));
+  expect(useSessionStore.getState().transcript.at(-1)?.text).toBe("hello");
+});
+
 test("closes the stream on unmount", () => {
   const { unmount } = renderHook(() => useSessionStream("s1"));
   const es = FakeEventSource.instances[0];

frontend/src/test/fakeEventSource.ts

@@ -4,15 +4,26 @@ export class FakeEventSource {
   onopen: (() => void) | null = null;
   onerror: (() => void) | null = null;
   closed = false;
+  private listeners = new Map<string, Set<(e: { data: string }) => void>>();
   constructor(public url: string) {
     FakeEventSource.instances.push(this);
   }
+  /** Dispatch to the default onmessage handler (unnamed `event: message`). */
   emit(obj: unknown) {
     this.onmessage?.({ data: JSON.stringify(obj) });
   }
-  // no-op stubs so useSessionStream's addEventListener calls don't throw
-  addEventListener(_type: string, _handler: unknown) {}
-  removeEventListener(_type: string, _handler: unknown) {}
+  /** Dispatch to handlers registered for a NAMED event (e.g. "ui_request"). */
+  emitNamed(type: string, obj: unknown) {
+    const data = JSON.stringify(obj);
+    for (const h of this.listeners.get(type) ?? []) h({ data });
+  }
+  addEventListener(type: string, handler: (e: { data: string }) => void) {
+    if (!this.listeners.has(type)) this.listeners.set(type, new Set());
+    this.listeners.get(type)!.add(handler);
+  }
+  removeEventListener(type: string, handler: (e: { data: string }) => void) {
+    this.listeners.get(type)?.delete(handler);
+  }
   close() {
     this.closed = true;
   }