refactor(gateway): unify control-ui and plugin webhook routing

src/gateway/control-ui-http-utils.ts

@@ -0,0 +1,19 @@
+import type { ServerResponse } from "node:http";
+
+export function isReadHttpMethod(method: string | undefined): boolean {
+  return method === "GET" || method === "HEAD";
+}
+
+export function respondPlainText(res: ServerResponse, statusCode: number, body: string): void {
+  res.statusCode = statusCode;
+  res.setHeader("Content-Type", "text/plain; charset=utf-8");
+  res.end(body);
+}
+
+export function respondNotFound(res: ServerResponse): void {
+  respondPlainText(res, 404, "Not Found");
+}
+
+export function respondMethodNotAllowed(res: ServerResponse): void {
+  respondPlainText(res, 405, "Method Not Allowed");
+}