Fix prerender tags when notFound is called (#74577)
This ensures we don't lose tags added to a page before `notFound()` is
called as otherwise a fetch on the page that has been revalidated won't
be able to be applied correctly if dropped from a `notFound()` call.
This was broken recently in the refactors of handling rendering in
app-render as it worked correctly in `v14`.
x-ref: [slack
thread](https://vercel.slack.com/archives/C04V3E1UYNQ/p1736200963684869)
Closes https://linear.app/vercel/issue/ENET-1381
Closes https://linear.app/vercel/issue/NEXT-3946
Closes https://github.com/vercel/next.js/issues/73973
Closes https://github.com/vercel/next.js/issues/66540
