[Flight][Reply] Close Response after creating root chunk (#27634)

creating the root after closing the response can lead to a promise that
never rejects. This is not intended use of the decodeReply API but if
pathalogical cases where you pass a raw FormData into this fucntion with
no zero chunk it can hang forever. This reordering causes a connection
error instead

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Co-authored-by: Zack Tanner <zacktanner@gmail.com>