Fix cache revalidation queued during streaming (#100448)
Next.js previously flushed revalidations after preparing the HTML stream
or running a Route Handler, before the response body finished streaming.
That flush dispatched queued tags and collected existing background
cache promises. Work added during streaming was omitted, so later cache
regeneration lacked `waitUntil` coverage and later tag invalidations
were never dispatched.
This extends the response-close approach proposed in #99772 beyond
dynamic HTML to Flight responses, Route Handlers, and streamed Server
Actions. `AfterContext` registers a `waitUntil` promise before the
handler returns and flushes newly queued work at close. Its snapshot
excludes earlier flushes and runs before `after()` callbacks start, so
their invalidations wait for callback completion. The change also
handles already-closed Node responses and Edge error paths that must
release close waiters. Server Action barriers retain their existing
ordering.
Fixes #99339
Fixes #99566
Closes #99772
