chore: fix failing CI tests (#19937)

## Background

Two independent fixes collided:

[PR #17782](https://github.com/vercel/ai/pull/17782) added support for
array-based thinking and text content. The tests correctly verify those
deltas are converted.

Later, [PR #19169](https://github.com/vercel/ai/pull/19169) made streams
without a finish reason produce an error.

The #17782 fixtures only sent `finish_reason: null`, then `[DONE]`. They
passed before #19169 but became invalid afterwards

## Summary

added a realistic final chunk with `finish_reason: 'stop'` before
`[DONE]`

## End-to-End Verification

na

## Checklist

- [x] All commits are signed (PRs with unsigned commits cannot be
merged)
- [x] Tests have been added / updated (for bug fixes / features)
- [ ] Documentation has been added / updated (for bug fixes / features)
- [ ] A _patch_ changeset for relevant packages has been added (for bug
fixes / features - run `pnpm changeset` in the project root)
- [x] I have reviewed this pull request (self-review)