fix(backend): bridge maps real Pi message_update->text_delta for FE streaming

Live end-to-end against real pi --mode rpc revealed Pi streams assistant text as
top-level message_update events whose nested assistantMessageEvent carries the
incremental delta — not the top-level text_delta the fake-pi-rpc emits. The bridge
now maps message_update(assistantMessageEvent.text_delta).delta -> FE text_delta,
so the chat shows the model's output during a real session.

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

backend/src/bridge/session-bridge.ts

@@ -19,6 +19,11 @@ export class SessionBridge {
         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 === "message_update" && m.assistantMessageEvent?.type === "text_delta") {
+        // Real Pi streams assistant text as message_update events whose nested
+        // assistantMessageEvent carries the incremental `delta`. Map it to the FE's
+        // text_delta contract. (Top-level text_delta below is what the fake-pi-rpc emits.)
+        this.fan({ type: "text_delta", text: m.assistantMessageEvent.delta ?? "" });
       } else if (m.type === "text_delta") {
         this.fan({ type: "text_delta", text: m.text ?? "" });
       } else if (m.type === "system_event") {

backend/test/session-bridge.test.ts

@@ -15,6 +15,19 @@ function fakeRpc() {
   };
 }
 
+test("real Pi message_update (assistantMessageEvent text_delta) becomes a text_delta to the FE", () => {
+  const { rpc, fire } = fakeRpc();
+  const b = new SessionBridge(rpc);
+  const seen: any[] = [];
+  b.onClientEvent((e) => seen.push(e));
+  // Shape emitted by real `pi --mode rpc`: text streams inside message_update.assistantMessageEvent.delta
+  fire({ type: "message_update", assistantMessageEvent: { type: "text_delta", contentIndex: 0, delta: "Avvio" } });
+  expect(seen[0]).toEqual({ type: "text_delta", text: "Avvio" });
+  // Non-text message_update events (e.g. text_start) are ignored.
+  fire({ type: "message_update", assistantMessageEvent: { type: "text_start", contentIndex: 0 } });
+  expect(seen).toHaveLength(1);
+});
+
 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);