fix: restore model activity reasoning stream

backend/test/session-bridge.test.ts

@@ -28,6 +28,20 @@ test("real Pi message_update (assistantMessageEvent text_delta) becomes a text_d
   expect(seen).toHaveLength(1);
 });
 
+test("real Pi thinking_delta becomes a dedicated activity_delta to the FE", () => {
+  const { rpc, fire } = fakeRpc();
+  const b = new SessionBridge(rpc);
+  const seen: any[] = [];
+  b.onClientEvent((e) => seen.push(e));
+
+  fire({
+    type: "message_update",
+    assistantMessageEvent: { type: "thinking_delta", contentIndex: 0, delta: "Valuto le ambiguità" },
+  });
+
+  expect(seen).toEqual([{ type: "activity_delta", text: "Valuto le ambiguità" }]);
+});
+
 test("extension_ui_request nativo (method:input, title=json) diventa ui_request col descriptor ed è il pendente", () => {
   const { rpc, fire } = fakeRpc();
   const b = new SessionBridge(rpc);