Fix notFound status code with ISR in app (#55542)
This ensures we properly set/restore the status code with ISR paths in
app router so that when we set the 404 status code with `notFound` it is
persisted properly.
Fixes: https://github.com/vercel/next.js/issues/43831
Closes: https://github.com/vercel/next.js/issues/48342
x-ref:
https://github.com/vercel/next.js/issues/49387#issuecomment-1722575969
