feat: add missing onAbort callback

[qkdreyer]

Background

For things such as token tracking, it would be helpful if callbacks can be registered on a per-call basis. See #11468

Summary

Add onAbort callback support to Agent.stream() (inspired by #11980)

[vercel]

Additional Suggestion:

The onAbort callback parameter is accepted in createAgentUIStream's type signature but silently dropped - it is never destructured or passed to agent.stream(), so abort callbacks never fire.

[qkdreyer]

@aayush-kapoor 👋

[qkdreyer]

cc @lgrammel 🙏