Fix hybrid Pages/App Router not-found rendering for adapter-hosted outputs.

When Cache Components is enabled and a Pages Router route resolves with `notFound`, Next.js can select the App Router `/_not-found` page. Adapter requests currently fail to complete that cross-router handoff when the App Router not-found page suspends, leaving request-time content out of the response.

Ensure this handoff returns a 404 containing the complete App Router not-found result, including per-request content behind Suspense. Separate requests must render their current request data rather than reuse an incomplete shell. Keep ordinary `next start` behavior on its normal PPR/resumption path unchanged. Also clarify the relevant request metadata documentation so resume requests with resumable state and resume requests requiring a full dynamic render are unambiguous.