fix: revert using server read for load function fetch (#12876)

* revert server read for fetch

* changeset

* check file existance instead

---------

Co-authored-by: Simon H <[email protected]>

Author: teemingc

.changeset/eight-insects-live.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: don't try reading assets directly that aren't present

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

@@ -97,7 +97,7 @@ export function create_fetch({ event, options, manifest, state, get_cookie_heade
 						return new Response(state.read(file), {
 							headers: type ? { 'content-type': type } : {}
 						});
-					} else if (read_implementation) {
+					} else if (read_implementation && file in manifest._.server_assets) {
 						const length = manifest._.server_assets[file];
 						const type = manifest.mimeTypes[file.slice(file.lastIndexOf('.'))];