feat add onParts support for chat API and processing functions

[NoelJacob]

Currently this is made for my personal use. It's a hook for any transformation when each part of stream is recieved. Also made prepareBody support async functions. Currently only onFilePart is added and only on react.

Would this feature be appreciated? If so I can make a proper PR.

[lgrammel]

thanks - in ai sdk 5 you can implement your own ChatTransport and transform incoming UIMessageStreamPart streams.

[NoelJacob]

I saw the blog but any docs for it? Where can learn what functions to use and stuff?

[zhm]

@NoelJacob I got a custom onPart working in v5 with this custom transport that overrides the default transport.

https://gist.github.com/zhm/ae4ede57ead9712820340e2f1248c21a

A few notes:

• adds onPart param to the chat store factory
• UIDataPartSchemas and InferUIDataParts types don't seem to be exported (yet), but I might be missing something
• Would be far less boilerplate if defaultChatStore() accepted an optional transport
• Perhaps onPart should just be baked into defaultChatStore with onFinish, seems fairly reasonable