fix(ai): change createAgentUIStream onStepFinish to UIMessageStreamOnStepFinishCallback

[sleitor]

Summary

Fixes #12841

The onStepFinish callback in createAgentUIStream, createAgentUIStreamResponse, and pipeAgentUIStreamToResponse was typed as ToolLoopAgentOnStepFinishCallback (LLM-level step result with token usage, finish reason, etc.), which is inconsistent with the UIMessageStreamOnStepFinishCallback (UI-level messages, isContinuation, responseMessage) used in createUIMessageStream.

This makes it impossible to use createAgentUIStream for the primary use-case of persisting intermediate messages during multi-step agent runs (as described in #12383).

Changes

• create-agent-ui-stream.ts: Changed onStepFinish type from ToolLoopAgentOnStepFinishCallback to UIMessageStreamOnStepFinishCallback; moved it from agent.stream() to result.toUIMessageStream()
• create-agent-ui-stream-response.ts: Same type change
• pipe-agent-ui-stream-to-response.ts: Same type change
• stream-text-result.ts: Added onStepFinish to UIMessageStreamOptions so it threads through toUIMessageStream(), pipeUIMessageStreamToResponse(), and toUIMessageStreamResponse()
• stream-text.ts: Destructured and passed onStepFinish to handleUIMessageStreamFinish() in toUIMessageStream()

Before / After

Before: onStepFinish in createAgentUIStream received OnStepFinishEvent with raw LLM details (usage tokens, finish reason, model response, etc.)

After: onStepFinish receives { messages, isContinuation, responseMessage } — the same shape as createUIMessageStream's onStepFinish, making it easy to persist intermediate UI messages:

await createAgentUIStream({
  agent,
  uiMessages,
  onStepFinish: async ({ messages, responseMessage }) => {
    await db.saveMessages(messages); // persist intermediate state
  },
});

[rovo89]

@sleitor I believe my approach is cleaner: #12843. Did you miss anything there that made you come up with your own PR?

[sleitor]

👋 Gentle ping — just checking if this is still on the radar for review. Happy to address any feedback or rebase if needed!