Strip the partial marker in createFetch
With Cache Components, the server prepends a byte that says whether a
response is partial. processFetch stripped it by rebuilding the Response
around the stripped stream, patching `url` and `redirected` back on, and
returned the flag separately as a `cacheData` promise.
createFetch now strips the byte itself, once, from the final response
after any redirect replay, and returns the stripped body with
`isPartial` on the RSCResponse. processFetch, the rebuilt Response and
`cacheData` are gone.
Server action redirects, the only other caller, strip the byte from the
response body and decode the stream directly. Other action responses are
decoded as before.
