Keep Fast mode on when you save a Discover session

## Summary

Discover sessions now persist ES|QL Fast mode (esqlApproximation) on save and restore it on open. There is no new toggle. The existing lightning-bolt Fast mode control is what gets stored. Docs already say Fast mode is preserved on dashboards. They do not say the same for Discover sessions. The Discover as code API also returns esql_approximate. That API is not on a published user page.

**Why this needs docs:** Fast mode docs already describe persistence for dashboards, so omitting Discover sessions is now incomplete.

## Resources

- PR [#282776](https://github.com/elastic/kibana/pull/282776) — [discover session] persist tab isApproximate
- Product issue: https://github.com/elastic/kibana/issues/275333

## Availability

| Channel | Details |
|---------|---------|
| **Stack** | v9.6.0 |
| **Serverless** | Aug 24–Aug 28 |
| **Feature flag** | None — active by default |

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*Created with [Docs Quest Scanner](https://github.com/florent-leborgne/docs-quest-scanner) by @florent-leborgne*

## Suggested edits

[Use ES|QL in the Kibana UI](https://www.elastic.co/docs/explore-analyze/query-filter/languages/esql-kibana) > Use Fast mode
- **What the docs say:** Fast mode is preserved when you save or share a dashboard.
- **What to add:** Add that Fast mode is also restored when you save and reopen a Discover session. Do not add a new control. The existing bolt Fast mode option is what is stored. Applies from 9.6.0 and in serverless.

[Save a Discover session for reuse](https://www.elastic.co/docs/explore-analyze/discover/save-open-search) > Save a Discover session
- **What the docs say:** By default, a Discover session stores the query text, filters, and current view of Discover, including the columns and sort order in the document table, and the data view.
- **What to add:** Add Fast mode to the stored-state list for ES|QL sessions. Opening the session should restore the bolt Fast mode setting. Applies from 9.6.0 and in serverless.