Harden the benchmark test fixture

Make fixture server startup and shutdown reject on errors instead of hanging
or resolving unconditionally.  Use the Node Buffer path when bridging fetch
responses back to the HTTP server.

#795 (comment)
#795 (comment)
#795 (comment)

Assisted-by: Codex:gpt-5.5

Author: dahlia

test/bench/fixture.ts

@@ -4,6 +4,7 @@ import {
   MemoryKvStore,
 } from "@fedify/fedify";
 import { Create, Endpoints, Person } from "@fedify/vocab";
+import { Buffer } from "node:buffer";
 import {
   createServer,
   type IncomingMessage,
@@ -80,13 +81,22 @@ export async function spawnBenchmarkTarget(): Promise<BenchmarkTargetFixture> {
       await writeFetchResponse(response, outgoing);
     },
   );
-  await new Promise<void>((resolve) => server.listen(0, "127.0.0.1", resolve));
+  await new Promise<void>((resolve, reject) => {
+    server.once("error", reject);
+    server.listen(0, "127.0.0.1", () => {
+      server.off("error", reject);
+      resolve();
+    });
+  });
   const address = server.address() as AddressInfo;
   return {
     url: new URL(`http://${address.address}:${address.port}/`),
     inboxUserAgent: () => inboxUserAgent,
     requests: () => requests.slice(),
-    close: () => new Promise<void>((resolve) => server.close(() => resolve())),
+    close: () =>
+      new Promise<void>((resolve, reject) => {
+        server.close((error) => error == null ? resolve() : reject(error));
+      }),
   };
 }
 
@@ -135,5 +145,5 @@ async function writeFetchResponse(
   response.headers.forEach((value, name) => {
     outgoing.setHeader(name, value);
   });
-  outgoing.end(new Uint8Array(await response.arrayBuffer()));
+  outgoing.end(Buffer.from(await response.arrayBuffer()));
 }