feat(backend): SSE hub with pending-widget re-emit on (re)subscribe

backend/src/sse/sse-hub.ts

@@ -0,0 +1,13 @@
+type Send = (event: string, data: object) => void;
+export class SseHub {
+  private subs = new Map<string, Set<Send>>();
+  subscribe(sessionId: string, send: Send, pending?: object | null): () => void {
+    if (!this.subs.has(sessionId)) this.subs.set(sessionId, new Set());
+    this.subs.get(sessionId)!.add(send);
+    if (pending) send("ui_request", { ui_request: pending });
+    return () => this.subs.get(sessionId)?.delete(send);
+  }
+  publish(sessionId: string, event: string, data: object): void {
+    for (const s of this.subs.get(sessionId) ?? []) s(event, data);
+  }
+}

backend/test/sse-hub.test.ts

@@ -0,0 +1,16 @@
+import { test, expect } from "vitest";
+import { SseHub } from "../src/sse/sse-hub.js";
+
+test("re-emette il widget pendente alla sottoscrizione", () => {
+  const hub = new SseHub(); const sent: any[] = [];
+  hub.subscribe("s1", (ev, data) => sent.push({ ev, data }), { id: "u1", widget: "select" });
+  expect(sent[0]).toEqual({ ev: "ui_request", data: { ui_request: { id: "u1", widget: "select" } } });
+});
+
+test("publish raggiunge i subscriber e unsubscribe li stacca", () => {
+  const hub = new SseHub(); const sent: any[] = [];
+  const off = hub.subscribe("s1", (ev, data) => sent.push({ ev, data }));
+  hub.publish("s1", "text_delta", { text: "x" });
+  off(); hub.publish("s1", "text_delta", { text: "y" });
+  expect(sent).toEqual([{ ev: "text_delta", data: { text: "x" } }]);
+});