fix(vercel): return real `turn.stream` to useChat #56

Job	Run time
test-integration node 20	1m 12s
test node 22	47s
test-integration node 24	1m 3s
demo demo/vercel/react/use-client-transport	1m 25s
check	31s
test node 20	43s
test node 24	44s
test-integration node 22	1m 13s
demo demo/vercel/react/use-chat	1m 38s
build	49s
	10m 5s

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