fix(studio): poll background generation and handle gateway failures

packages/design-studio/test/generation-transport.test.ts

@@ -0,0 +1,66 @@
+import { expect, test } from "bun:test";
+import { generationResponse, requestGeneration } from "../src/generation-transport.ts";
+
+test("HTML gateway failures produce a clear diagnostic rather than a JSON syntax error", async () => {
+  for (const status of [502, 503, 504])
+    await expect(
+      generationResponse(new Response("<html>Gateway error</html>", { status })),
+    ).rejects.toThrow(`HTTP ${status}`);
+  await expect(
+    generationResponse(new Response("<html>private proxy text</html>", { status: 504 })),
+  ).rejects.not.toThrow("private proxy text");
+});
+test("generation uses short job polls and preserves legacy response compatibility", async () => {
+  const batch = { alternatives: [1, 2, 3], diagnostics: [] };
+  let calls = 0;
+  const transport = (async (_url: unknown, init?: RequestInit) => {
+    calls++;
+    if (calls === 1) {
+      expect(new Headers(init?.headers).get("prefer")).toBe("respond-async");
+      return Response.json(
+        { job: { id: "11111111-1111-4111-8111-111111111111" } },
+        { status: 202 },
+      );
+    }
+    expect(init?.method).toBeUndefined();
+    return Response.json({ status: "complete", result: batch });
+  }) as unknown as typeof fetch;
+  expect(
+    await requestGeneration(
+      new URL("https://example.test/design-studio/api/generate"),
+      {},
+      new AbortController().signal,
+      transport,
+    ),
+  ).toEqual(batch);
+  expect(calls).toBe(2);
+  expect(
+    await requestGeneration(
+      new URL("https://example.test/api/generate"),
+      {},
+      new AbortController().signal,
+      (async () => Response.json(batch)) as unknown as typeof fetch,
+    ),
+  ).toEqual(batch);
+});
+test("cancelling a pending job sends cancellation without replacing a draft", async () => {
+  const controller = new AbortController();
+  let cancelled = false;
+  const transport = (async (_url: unknown, init?: RequestInit) => {
+    if (init?.method === "DELETE") {
+      cancelled = true;
+      return Response.json({ status: "cancelled" });
+    }
+    controller.abort();
+    return Response.json({ job: { id: "11111111-1111-4111-8111-111111111111" } }, { status: 202 });
+  }) as unknown as typeof fetch;
+  await expect(
+    requestGeneration(
+      new URL("https://example.test/api/generate"),
+      {},
+      controller.signal,
+      transport,
+    ),
+  ).rejects.toBeDefined();
+  expect(cancelled).toBe(true);
+});