Migrate app route to handler interface (#80008)
Continuation of https://github.com/vercel/next.js/pull/78166 this
implements the handler interface for app router route handlers. This
does still return the cache entry for ISR route handlers for the
incremental cache to handle but response handling is moved into the
handler here.
Validated against deploy tests
https://github.com/vercel/vercel/pull/13413 and
https://github.com/vercel/next.js/actions/runs/15359464126/job/43224461919
