Align the Vercel UI message stream with the data stream protocol - #906

Currently the Vercel protocol stream drops information the AI SDK client is built to consume. A completed run looks like this on the wire:

data: {"type":"start","messageId":"019..."}
data: {"type":"text-delta","id":"msg_1","delta":"Hello."}
data: {"type":"finish"}   // usage and finish reason are thrown away
data: [DONE]

And when a provider fails after streaming has started, the connection just drops, so useChat sits on a dead spinner with no error state.

After this PR, the same run emits every part the client processes, validated field by field against the UIMessageChunk schema in vercel/ai:

data: {"type":"start","messageId":"019..."}
data: {"type":"start-step"}
data: {"type":"text-delta","id":"msg_1","delta":"Hello."}
data: {"type":"finish-step"}
data: {"type":"finish","finishReason":"stop","messageMetadata":{"usage":{"prompt_tokens":10,...}}}
data: [DONE]

On the client that means message.metadata.usage is populated, finishReason lands in the chat state, and multi-step tool runs get proper step boundaries (start-step also makes the client insert its step-start parts, and finish-step resets its active text and reasoning state between steps, which guards against text block id reuse across steps).

A mid-stream failure now closes the stream properly instead of severing it:

data: {"type":"start","messageId":"019..."}
data: {"type":"text-delta","id":"msg_1","delta":"Hel"}
data: {"type":"error","errorText":"Provider connection lost."}   // client renders its error state
data: [DONE]

The exception is still reported to the handler, it just no longer escapes into an already-open response.