Ensure Undici is enabled for appDir in all cases (#42382)

Fixes
#41745 (comment)

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## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the
feature request has been accepted for implementation before opening a
PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing
doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

Co-authored-by: JJ Kasper <[email protected]>

packages/next/server/config.ts

@@ -65,9 +65,7 @@ export function setHttpClientAndAgentOptions(config: NextConfig) {
       )
     } else {
       // When appDir is enabled undici is the default because of Response.clone() issues in node-fetch
-      ;(global as any).__NEXT_USE_UNDICI = config.experimental?.appDir
-        ? true
-        : config.experimental?.enableUndici
+      ;(global as any).__NEXT_USE_UNDICI = config.experimental?.enableUndici
     }
   } else if (config.experimental?.enableUndici) {
     Log.warn(
@@ -260,6 +258,10 @@ function assignDefaults(dir: string, userConfig: { [key: string]: any }) {
     )
   }
 
+  if (result.experimental?.appDir) {
+    result.experimental.enableUndici = true
+  }
+
   if (result.basePath !== '') {
     if (result.basePath === '/') {
       throw new Error(

packages/next/server/next-server.ts

@@ -99,6 +99,7 @@ import { IncrementalCache } from './lib/incremental-cache'
 import { normalizeAppPath } from '../shared/lib/router/utils/app-paths'
 
 import { renderToHTMLOrFlight as appRenderToHTMLOrFlight } from './app-render'
+import { setHttpClientAndAgentOptions } from './config'
 
 export * from './base-server'
 
@@ -260,6 +261,8 @@ export default class NextNodeServer extends BaseServer {
       const { AsyncLocalStorage } = require('async_hooks')
       ;(global as any).AsyncLocalStorage = AsyncLocalStorage
     }
+    // ensure options are set when loadConfig isn't called
+    setHttpClientAndAgentOptions(this.nextConfig)
   }
 
   private compression = this.nextConfig.compress