)

## Summary

Fixes token count display showing "NaN / 200,000 tokens sent" in the
Search Playground after the AI SDK v5 upgrade.

The issue was on the client-side in `parseDataStream` (`api.ts`), where
`messageAnnotations` was being reset to `undefined` when a `text-start`
chunk was received. In AI SDK v5, annotations (including token counts)
are sent *before* the `text-start` chunk, so this reset was clearing the
already-collected token data. This PR removes that reset to preserve
annotations across the stream, ensuring token counts are correctly
attached to the final message and displayed in the UI.

### Before
<img width="865" height="502" alt="Screenshot 2025-12-16 at 10 39 42 AM"
src="https://github.com/user-attachments/assets/331d3c7c-1b3e-455e-b8d7-561e2832054c"
/>

### After
<img width="1008" height="519" alt="Screenshot 2025-12-16 at 9 56 13 AM"
src="https://github.com/user-attachments/assets/ec507093-e382-4320-b610-cc3d53a74c3b"
/>

### Checklist

Check the PR satisfies following conditions.

Reviewers should verify this PR satisfies this list as well.

- [ ] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/src/platform/packages/shared/kbn-i18n/README.md)
- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [ ] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)
- [ ] This was checked for breaking HTTP API changes, and any breaking
changes have been approved by the breaking-change committee. The
`release_note:breaking` label should be applied in these situations.
- [ ] [Flaky Test
Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on any tests changed
- [x] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)
- [x] Review the [backport
guidelines](https://docs.google.com/document/d/1VyN5k91e5OVumlc0Gb9RPa3h1ewuPE705nRtioPiTvY/edit?usp=sharing)
and apply applicable `backport:*` labels.

### Identify risks

Does this PR introduce any risks? For example, consider risks like hard
to test bugs, performance regression, potential of data loss.

Describe the risk, its severity, and mitigation for each identified
risk. Invite stakeholders and evaluate how to proceed before merging.

- [ ] [See some risk
examples](https://github.com/elastic/kibana/blob/main/RISK_MATRIX.mdx)
- [ ] ...

## Release note
Fixes token count display showing "NaN" in Search Playground by
preserving message annotations across the AI SDK v5 stream.

(cherry picked from commit 6eae531)