[backport] Ensure setAssetPrefix updates config instance (#82165)

Backports #82160 to our next-15-4
release branch

Author: ijjk

packages/next/src/server/base-server.ts

@@ -1707,7 +1707,8 @@ export default abstract class Server<
   ): Promise<void>
 
   public setAssetPrefix(prefix?: string): void {
-    this.renderOpts.assetPrefix = prefix ? prefix.replace(/\/$/, '') : ''
+    this.nextConfig.assetPrefix = prefix ? prefix.replace(/\/$/, '') : ''
+    this.renderOpts.assetPrefix = this.nextConfig.assetPrefix
   }
 
   protected prepared: boolean = false

test/e2e/app-dir/asset-prefix-absolute/asset-prefix-absolute-no-path.test.ts

@@ -0,0 +1,32 @@
+import { nextTestSetup } from 'e2e-utils'
+
+describe('app-dir absolute assetPrefix', () => {
+  const { next } = nextTestSetup({
+    files: __dirname,
+    nextConfig: {
+      assetPrefix: 'https://example.vercel.sh/',
+    },
+  })
+
+  it('bundles should return 200 on served assetPrefix', async () => {
+    const $ = await next.render$('/')
+
+    let bundles = []
+    for (const script of $('script').toArray()) {
+      const { src } = script.attribs
+      if (src?.includes('https://example.vercel.sh/_next/static')) {
+        bundles.push(src)
+      }
+    }
+
+    expect(bundles.length).toBeGreaterThan(0)
+
+    for (const src of bundles) {
+      // Remove hostname to check if pathname is still used for serving the bundles
+      const bundlePathWithoutHost = decodeURI(new URL(src).pathname)
+      const { status } = await next.fetch(bundlePathWithoutHost)
+
+      expect(status).toBe(200)
+    }
+  })
+})

test/e2e/app-dir/asset-prefix-absolute/asset-prefix-absolute.test.ts

@@ -3,6 +3,9 @@ import { nextTestSetup } from 'e2e-utils'
 describe('app-dir absolute assetPrefix', () => {
   const { next } = nextTestSetup({
     files: __dirname,
+    nextConfig: {
+      assetPrefix: 'https://example.vercel.sh/custom-asset-prefix',
+    },
   })
 
   it('bundles should return 200 on served assetPrefix', async () => {