fix: set revalidate cache header on 404'd static assets (#12530)

* fix: set revalidate cache header on 404'd static assets

Some adapters (like Vercel) apply a immutable cache header by default to static assets. They respect explicitly set headers though. Therefore apply a "must immediately revalidate" cache header to the 404 response of static assets in the SvelteKit runtime

related to #9089

* Update packages/kit/src/runtime/server/respond.js

Co-authored-by: Conduitry <git@chor.date>

---------

Co-authored-by: Conduitry <git@chor.date>

.changeset/light-pears-help.md

@@ -0,0 +1,5 @@
+---
+"@sveltejs/kit": patch
+---
+
+fix: set revalidate cache header on 404'd static assets

packages/kit/src/runtime/server/respond.js

@@ -116,7 +116,10 @@ export async function respond(request, options, manifest, state) {
 	}
 
 	if (decoded.startsWith(`/${options.app_dir}`)) {
-		return text('Not found', { status: 404 });
+		// Ensure that 404'd static assets are not cached - some adapters might apply caching by default
+		const headers = new Headers();
+		headers.set('cache-control', 'public, max-age=0, must-revalidate');
+		return text('Not found', { status: 404, headers });
 	}
 
 	const is_data_request = has_data_suffix(decoded);