Strip completeness marker from server action redirect responses
Server action redirects to fully static pages include the prerendered
flight data for the redirect target, which has the
`ResponseCompletenessMarker` byte prepended at build time. The server
action reducer passed this response directly to React's
`createFromFetch` without stripping the marker, causing a `Connection
closed` error because the marker byte is not valid RSC Flight data.
Fix by running the response through `processFetch` (which calls
`stripCompletenessMarker`) before passing it to `createFromFetch`. Also
export `processFetch` so it can be reused, and preserve the `redirected`
property on the reconstructed `Response` since the `Response`
constructor does not carry it over from the original.
