data part is not appended to message

[JerryKwan]

Description

When using DataStreamWriter's writeData() to send custom data, the data part is not assigned the message as the operations when invoking writeMessageAnnotation() which annotations are assigned to message as expected.
In the current implementation the data is cached and returned as data for the whole chat, not accessible by message, although Message has a data field, but it's never assigned.
As you can see from the following code snippets.

https://github.com/vercel/ai/blob/main/packages/ui-utils/src/process-chat-response.ts#L222-L234
https://github.com/vercel/ai/blob/main/packages/ui-utils/src/types.ts#L47-L82

It would be better the data part is accessible by message like annotations.

Code example

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AI provider

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Additional context

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