Bugfix: Fix #15004 by applying MDN recommended fix when proxying Response in page_load (#15005)

* Apply MDN recommended fix when proxying Response to avoid TypeError.

On Node v24+, the undici library used for HTTP has a Response class with
a private element. Proxying the response therefore fails when calling
clone() as the method returned by the proxy cannot access the private
state element.

* Use more comprehensive Response proxy fix that makes linter happy.

By using Object.defineProperty(), we make the function identical to
introspection. Also, the linter didn't like the `...args`.

* Add page to test undici Response TypeError.

* Add generated changeset file.

* Apply suggestion from @elliott-with-the-longest-name-on-github

---------

Co-authored-by: Elliott Johnson <hello@ell.iott.dev>

Author: drewsilcock

.changeset/tangy-donkeys-knock.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: TypeError when doing response.clone() in page load

packages/kit/src/runtime/server/page/load_data.js

@@ -316,7 +316,7 @@ export function create_universal_fetch(event, state, fetched, csr, resolve_opts)
 		let teed_body;
 
 		const proxy = new Proxy(response, {
-			get(response, key, _receiver) {
+			get(response, key, receiver) {
 				/**
 				 * @param {string | undefined} body
 				 * @param {boolean} is_b64
@@ -427,7 +427,28 @@ export function create_universal_fetch(event, state, fetched, csr, resolve_opts)
 					};
 				}
 
-				return Reflect.get(response, key, response);
+				const value = Reflect.get(response, key, response);
+
+				if (value instanceof Function) {
+					// On Node v24+, the Response object has a private element #state – we
+					// need to bind this function to the response in order to allow it to
+					// access this private element. Defining the name and length ensure it
+					// is identical to the original function when introspected.
+					return Object.defineProperties(
+						/**
+						 * @this {any}
+						 */
+						function () {
+							return Reflect.apply(value, this === receiver ? response : this, arguments);
+						},
+						{
+							name: { value: value.name },
+							length: { value: value.length }
+						}
+					);
+				}
+
+				return value;
 			}
 		});
 

packages/kit/test/apps/basics/src/routes/load/server-response-clone/+page.js

@@ -0,0 +1,9 @@
+/** @type {import('@sveltejs/kit').Load} */
+export async function load({ url, fetch }) {
+	const href = new URL(url);
+	href.pathname = '';
+
+	const res = await fetch(href);
+	const clonedRes = res.clone();
+	return { text: await clonedRes.text() };
+}

packages/kit/test/apps/basics/src/routes/load/server-response-clone/+page.svelte

@@ -0,0 +1,10 @@
+<script>
+	/** @type {import('./$types').PageData} */
+	export let data;
+</script>
+
+<h1>the text is:</h1>
+
+<p>
+	{data.text}
+</p>