fix: harden resume and SSE replay

backend/test/sse-route.test.ts

@@ -0,0 +1,82 @@
+import { test, expect } from "vitest";
+import { buildApp } from "../src/app.js";
+import { loadConfig } from "../src/config.js";
+import { SseHub } from "../src/sse/sse-hub.js";
+
+async function readUntil(
+  reader: ReadableStreamDefaultReader<Uint8Array>,
+  predicate: (text: string) => boolean,
+): Promise<string> {
+  const decoder = new TextDecoder();
+  let text = "";
+  const deadline = Date.now() + 1_000;
+  while (Date.now() < deadline) {
+    const result = await Promise.race([
+      reader.read(),
+      new Promise<{ timeout: true }>((resolve) =>
+        setTimeout(() => resolve({ timeout: true }), Math.max(0, deadline - Date.now())),
+      ),
+    ]);
+    if ("timeout" in result || result.done) break;
+    text += decoder.decode(result.value);
+    if (predicate(text)) break;
+  }
+  return text;
+}
+
+async function captureReplay(options: { query?: string; lastEventId?: string }): Promise<string> {
+  const hub = new SseHub();
+  hub.publish("s1", "info", { type: "info", text: "one" });
+  hub.publish("s1", "info", { type: "info", text: "two" });
+  hub.publish("s1", "info", { type: "info", text: "three" });
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "/tmp/h" }), {
+    hub,
+    thtRunner: {} as any,
+  });
+  await app.listen({ port: 0, host: "127.0.0.1" });
+  const port = (app.server.address() as { port: number }).port;
+  const controller = new AbortController();
+
+  try {
+    const response = await fetch(
+      `http://127.0.0.1:${port}/sessions/s1/events${options.query ?? ""}`,
+      {
+        headers: options.lastEventId ? { "Last-Event-ID": options.lastEventId } : undefined,
+        signal: controller.signal,
+      },
+    );
+    const reader = response.body!.getReader();
+    const body = await readUntil(reader, (text) => text.includes('"three"'));
+    await reader.cancel();
+    return body;
+  } finally {
+    controller.abort();
+    await app.close();
+  }
+}
+
+test("SSE emits ids and honors the native Last-Event-ID replay cursor", async () => {
+  const body = await captureReplay({ lastEventId: "1" });
+
+  expect(body).not.toContain('"one"');
+  expect(body).toContain("id: 2\nevent: info\n");
+  expect(body).toContain('data: {"type":"info","text":"two"}');
+  expect(body).toContain("id: 3\nevent: info\n");
+  expect(body).toContain('data: {"type":"info","text":"three"}');
+});
+
+test("SSE honors the manual lastEventId query cursor", async () => {
+  const body = await captureReplay({ query: "?lastEventId=2" });
+
+  expect(body).not.toContain('"one"');
+  expect(body).not.toContain('"two"');
+  expect(body).toContain("id: 3\nevent: info\n");
+  expect(body).toContain('data: {"type":"info","text":"three"}');
+});
+
+test("SSE uses the newer valid cursor when header and query are both present", async () => {
+  const body = await captureReplay({ query: "?lastEventId=2", lastEventId: "1" });
+
+  expect(body).not.toContain('"two"');
+  expect(body).toContain("id: 3\nevent: info\n");
+});