fix: avoid unknown config option Vite warning (#15451)

Co-authored-by: Ben McCann <322311+benmccann@users.noreply.github.com>
Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>

Author: 3 people

.changeset/lazy-shirts-attend.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: avoid Vite warning about unknown `codeSplitting` option

packages/kit/src/exports/vite/index.js

@@ -932,9 +932,7 @@ async function kit({ svelte_config }) {
 								assetFileNames: `${prefix}/assets/[name].[hash][extname]`,
 								hoistTransitiveImports: false,
 								sourcemapIgnoreList,
-								inlineDynamicImports: is_rolldown ? undefined : !split,
-								// @ts-ignore: only available in Vite 8
-								codeSplitting: is_rolldown ? split : undefined
+								inlineDynamicImports: is_rolldown ? undefined : !split
 							},
 							preserveEntrySignatures: 'strict',
 							onwarn(warning, handler) {
@@ -967,6 +965,13 @@ async function kit({ svelte_config }) {
 						}
 					}
 				};
+
+				// we must reference Vite 8 options conditionally. Otherwise, older Vite
+				// versions throw an error about unknown config options
+				if (is_rolldown && new_config?.build?.rollupOptions?.output) {
+					// @ts-ignore only available in Vite 8
+					new_config.build.rollupOptions.output.codeSplitting = split;
+				}
 			} else {
 				new_config = {
 					appType: 'custom',