feat: harden runtime readiness and session workflow

frontend/src/api/sessions.test.ts

@@ -1,6 +1,6 @@
 import { http, HttpResponse } from "msw";
 import { server } from "../test/msw";
-import { createSession, listSessions } from "./sessions";
+import { createSession, listSessions, prewarmRuntime } from "./sessions";
 import {
   renameSession, setSessionGroup, archiveSession, unarchiveSession,
   deleteSession, getSessionDocuments,
@@ -18,6 +18,18 @@ test("createSession POSTs only {question} and returns the id", async () => {
   expect(body).toEqual({ question: "q" });
 });
 
+test.each([202, 204])("prewarmRuntime accepts a body-less %s response", async (status) => {
+  let called = false;
+  server.use(
+    http.post("http://localhost:8787/runtime/prewarm", () => {
+      called = true;
+      return new HttpResponse(null, { status });
+    }),
+  );
+  await expect(prewarmRuntime()).resolves.toBeUndefined();
+  expect(called).toBe(true);
+});
+
 test("listSessions GETs the array", async () => {
   server.use(http.get("http://localhost:8787/sessions", () => HttpResponse.json([{ id: "s1", status: "open", question: "q", summary: null, created_at: "t", updated_at: null, author: null }])));
   const rows = await listSessions();