chore (ai): Add finishReason as a promise on StreamObjectResult to match StreamTextResult

[danielamitay]

@lgrammel Please take a look! This pull request simply adds Promise<FinishReason> to the StreamObject result to match how it is in StreamText, for parity/to make it easier to use the finish reason when streaming an object.

Background

• Promise<FinishReason> is an available property on StreamText API, but not StreamObject API.
• Without this change, devs need to observe the full stream instead of just await'ing a promise

Summary

• Update StreamObjectResult interface
• Add implementation bookkeeping for finishReasonPromise

Verification

• Finish reason promise is implemented exactly the same as other promises for streamObject
• Finish reason promise logic is the same as the promise in streamText
• Using this new promise, it resolves as expected with the other promises

[danielamitay]

Let me know if there is something I am missing or if there is a process to getting reviewed that I am unaware of 🙏