refactor(test): dedupe agent and discord test fixtures

src/discord/test-http-helpers.ts

@@ -0,0 +1,10 @@
+export function jsonResponse(body: unknown, status = 200): Response {
+  return new Response(JSON.stringify(body), { status });
+}
+
+export function urlToString(url: Request | URL | string): string {
+  if (typeof url === "string") {
+    return url;
+  }
+  return "url" in url ? url.url : String(url);
+}