feat: harden runtime readiness and session workflow

frontend/src/shell/AppShell.new-session.test.tsx

@@ -0,0 +1,89 @@
+import { render, screen, waitFor } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { http, HttpResponse, delay } from "msw";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+import { server } from "../test/msw";
+import { FakeEventSource } from "../test/fakeEventSource";
+import { useSessionStore } from "../store/sessionStore";
+import { AppShell } from "./AppShell";
+
+function renderShell() {
+  const client = new QueryClient({ defaultOptions: { queries: { retry: false } } });
+  return render(<QueryClientProvider client={client}><AppShell /></QueryClientProvider>);
+}
+
+beforeEach(() => {
+  FakeEventSource.instances = [];
+  (globalThis as any).EventSource = FakeEventSource;
+  useSessionStore.getState().resetSession();
+  server.use(
+    http.get("http://localhost:8787/sessions", () => HttpResponse.json([])),
+    http.get("http://localhost:8787/settings", () =>
+      HttpResponse.json({ workspace: "default", provider: "test", model: "test", thinking: "low" })),
+    http.get("http://localhost:8787/workspaces", () => HttpResponse.json([])),
+    http.get("http://localhost:8787/models", () => HttpResponse.json({ models: [] })),
+  );
+});
+
+test("New session starts prewarm without delaying composer focus", async () => {
+  let prewarmStarted = false;
+  server.use(
+    http.post("http://localhost:8787/runtime/prewarm", async () => {
+      prewarmStarted = true;
+      await delay(100);
+      return new HttpResponse(null, { status: 202 });
+    }),
+  );
+  renderShell();
+
+  await userEvent.click(screen.getByRole("button", { name: /^new session$/i }));
+
+  const composer = screen.getByRole("textbox", { name: /new question/i });
+  await waitFor(() => expect(prewarmStarted).toBe(true));
+  await waitFor(() => expect(composer).toHaveFocus());
+});
+
+test("shows the question and creation timer before POST /sessions resolves, then opens SSE", async () => {
+  let releaseCreate!: () => void;
+  const createMayFinish = new Promise<void>((resolve) => { releaseCreate = resolve; });
+  server.use(
+    http.post("http://localhost:8787/sessions", async () => {
+      await createMayFinish;
+      return HttpResponse.json({ id: "s-new" });
+    }),
+  );
+  renderShell();
+
+  const composer = screen.getByRole("textbox", { name: /new question/i });
+  await userEvent.type(composer, "How many patients?");
+  await userEvent.click(screen.getByRole("button", { name: /send/i }));
+
+  expect(await screen.findByText("How many patients?", { selector: "p" })).toBeInTheDocument();
+  expect(screen.getByText("Creating session…")).toBeInTheDocument();
+  expect(screen.getByText(/\d+s/)).toBeInTheDocument();
+  expect(FakeEventSource.instances).toHaveLength(0);
+
+  releaseCreate();
+  await waitFor(() => expect(FakeEventSource.instances).toHaveLength(1));
+  expect(screen.getByText("How many patients?", { selector: "p" })).toBeInTheDocument();
+  expect(screen.getByText("Analyzing question…")).toBeInTheDocument();
+  expect(useSessionStore.getState().currentPhase).toBe("F1");
+});
+
+test("a failed create restores the landing view and preserves the question for retry", async () => {
+  server.use(
+    http.post("http://localhost:8787/sessions", () =>
+      new HttpResponse("unavailable", { status: 503 })),
+  );
+  renderShell();
+
+  const composer = screen.getByRole("textbox", { name: /new question/i });
+  await userEvent.type(composer, "Keep this question");
+  await userEvent.click(screen.getByRole("button", { name: /send/i }));
+
+  expect(await screen.findByText(/failed to create session/i)).toBeInTheDocument();
+  expect(screen.getByText(/type your question/i)).toBeInTheDocument();
+  expect(composer).toHaveValue("Keep this question");
+  expect(useSessionStore.getState().lastUserEntry).toBeNull();
+  expect(FakeEventSource.instances).toHaveLength(0);
+});