feat(backend): SessionBridge widget-descriptor <-> RPC + pending widget

Decodes native extension_ui_request (method:input, title=JSON) into
ui_request ClientEvent; encodes respond() as extension_ui_response with
value payload; tracks pending widget; handles notify→info and steer.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/bridge/session-bridge.ts

@@ -0,0 +1,43 @@
+import type { RpcClient } from "../rpc/rpc-client.js";
+
+export type ClientEvent =
+  | { type: "ui_request"; ui_request: any }
+  | { type: "text_delta"; text: string }
+  | { type: "info"; [k: string]: any }
+  | { type: "system_event"; [k: string]: any };
+
+export class SessionBridge {
+  private pending: any = null;
+  private cbs = new Set<(e: ClientEvent) => void>();
+
+  constructor(private rpc: RpcClient) {
+    rpc.on("event", (m) => {
+      if (m.type === "extension_ui_request" && m.method === "input") {
+        let descriptor: any;
+        try { descriptor = JSON.parse(m.title); } catch { return; }
+        this.pending = descriptor;
+        this.fan({ type: "ui_request", ui_request: descriptor });
+      } else if (m.type === "extension_ui_request" && m.method === "notify") {
+        this.fan({ type: "info", level: m.notifyType ?? "info", text: m.message ?? "" });
+      } else if (m.type === "text_delta") {
+        this.fan({ type: "text_delta", text: m.text ?? "" });
+      } else if (m.type === "system_event") {
+        this.fan(m as ClientEvent);
+      }
+      // altri method nativi (setStatus/setWidget) e altri eventi Pi ignorati in MVP
+    });
+  }
+
+  private fan(e: ClientEvent) { for (const cb of this.cbs) cb(e); }
+
+  onClientEvent(cb: (e: ClientEvent) => void): void { this.cbs.add(cb); }
+
+  respond(uiResponse: object & { id: string }): void {
+    this.rpc.send({ type: "extension_ui_response", id: uiResponse.id, value: JSON.stringify(uiResponse) });
+    if (this.pending && uiResponse.id === this.pending.id) this.pending = null;
+  }
+
+  steer(text: string): void { this.rpc.send({ type: "steer", message: text }); }
+
+  pendingWidget(): object | null { return this.pending; }
+}

backend/test/session-bridge.test.ts

@@ -0,0 +1,42 @@
+import { test, expect, vi } from "vitest";
+import { SessionBridge } from "../src/bridge/session-bridge.js";
+
+function fakeRpc() {
+  const sent: any[] = [];
+  let evcb: any;
+  return {
+    rpc: {
+      send: (c: any) => sent.push(c),
+      on: (_: any, cb: any) => { evcb = cb; },
+      request: vi.fn(),
+    } as any,
+    sent,
+    fire: (m: any) => evcb(m),
+  };
+}
+
+test("extension_ui_request nativo (method:input, title=json) diventa ui_request ed è il pendente", () => {
+  const { rpc, fire } = fakeRpc();
+  const b = new SessionBridge(rpc);
+  const seen: any[] = [];
+  b.onClientEvent((e) => seen.push(e));
+  const descriptor = { id: "u1", widget: "select" };
+  fire({ type: "extension_ui_request", id: "u1", method: "input", title: JSON.stringify(descriptor) });
+  expect(seen[0]).toEqual({ type: "ui_request", ui_request: descriptor });
+  expect(b.pendingWidget()).toEqual(descriptor);
+});
+
+test("respond invia extension_ui_response con payload in value e azzera il pendente", () => {
+  const { rpc, sent, fire } = fakeRpc();
+  const b = new SessionBridge(rpc);
+  fire({ type: "extension_ui_request", id: "u1", method: "input", title: JSON.stringify({ id: "u1", widget: "select" }) });
+  b.respond({ id: "u1", choices: ["a"] });
+  expect(sent.at(-1)).toEqual({ type: "extension_ui_response", id: "u1", value: JSON.stringify({ id: "u1", choices: ["a"] }) });
+  expect(b.pendingWidget()).toBeNull();
+});
+
+test("steer invia un comando steer", () => {
+  const { rpc, sent } = fakeRpc();
+  new SessionBridge(rpc).steer("considera solo il 2024");
+  expect(sent.at(-1)).toEqual({ type: "steer", message: "considera solo il 2024" });
+});