Add support for Vercel AI SDK

https://vercel.com/docs/ai-sdk

> Hey @NathanTarbert, can we use this same ticket? Granted we should rename the issue, maybe "Add support for vercel AI SDK"
>
> I'm working on an implementation internally we might contribute if we get it working.
>
> Additionally there seems to be already an implementation for vercel ai sdk in this repository https://github.com/ag-ui-protocol/ag-ui/tree/main/integrations/vercel-ai-sdk/typescript, although it seems it's not published on npm https://www.npmjs.com/search?q=%40ag%2Dui%2Fvercel%2Dai%2Dsdk, and seems to be targeting an older version of vercel ai sdk (v4, latest version is v6)

_Originally posted by @cctidal in [#859](https://github.com/ag-ui-protocol/ag-ui/issues/859#issuecomment-3744928622)_