Avoid using unclosing prefetch streams in the browser (#89610)
Previously, `createPrefetchResponseStream` intentionally never called
`controller.close()` on the wrapper stream, to prevent React Flight from
erroring on unresolved references (dynamic holes). However, Chrome and
Firefox keep unclosed ReadableStreams with pending reads as native GC
roots, preventing the stream — and the entire FlightResponse captured in
the `reader.read().then(progress)` closure chain — from being
garbage-collected.
Now that React Flight supports `unstable_allowPartialStream` as an
option for `createFromReadableStream` (facebook/react#35731), we can
close the stream normally. Flight will mark unresolved chunks as
"halted" instead of erroring, which is the correct behavior for prefetch
responses.
Also removes the now-unnecessary `createUnclosingPrefetchStream` wrapper
from the legacy prefetch path in `fetch-server-response.ts`.
closes #89485
