x-ref: https://github.com/vercel/next.js/issues/30424#issuecomment-955615781
This fix the custom 404 is not rendering properly and can’t be built in web runtime when `concurrentFeatures` is enabled. We force 404 page to be rendered outside of middleware ssr. Then it could be the real fallback 404 page in next-server when any routes is not macthed.
Will check 500 related after #31057 is landed.
## Bug
- [x] Related to #30989
- [x] Integration tests added
e029ace5ed