Datafeed JSON editor suggests project_routing for cross-project search

## Summary

The JSON schemas that power autocomplete in the anomaly detection job wizard's JSON view and the edit datafeed flyout now include a `project_routing` string property. When you edit a datafeed as JSON, the editor suggests `project_routing` with a description explaining that it limits which linked projects the datafeed searches when cross-project search is enabled. The change is a surgical schema patch: Elasticsearch gates the field behind the serverless cross-project search feature flag and rejects it when CPS is not enabled for datafeeds.

**Why this needs docs:** The change only adds an autocomplete suggestion for a field that Elasticsearch still gates behind a feature flag, and the published CPS docs correctly state that anomaly detection jobs are not supported with CPS during technical preview, so nothing in the docs is wrong yet. Confidence stays under 0.8 because the feature is flag-gated and the limitation statement will need updating when datafeed CPS ships.

## Resources

- PR [#288268](https://github.com/elastic/kibana/pull/288268) — Add project_routing to ML datafeed JSON editor schemas

## Availability

| Channel | Details |
|---------|---------|
| **Stack** | v9.6.0 |
| **Serverless** | Sep 7–Sep 11 |
| **Feature status** | preview |
| **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*

## If you chose to document this

[Cross-project search](https://www.elastic.co/docs/explore-analyze/cross-project-search) > Limitations
- **What the docs say:** Anomaly detection: During technical preview, ML anomaly detection jobs are not supported with CPS. They continue to run on origin project data only.
- **What to add:** No change needed now. When Elasticsearch enables `project_routing` for datafeeds (elasticsearch #141828, elasticsearch-specification #6611 and #6616), revise this limitation and add a short paragraph to the Datafeeds section of the Run a job page explaining that a datafeed searches all linked projects by default and that `project_routing` (for example `_alias:_origin` or `_alias:prod-*`) limits the scope, editable in the datafeed JSON editor. Also revisit the Observability CPS page statement that machine learning rules are not available in CPS. Serverless only (technical preview, behind the cross-project search feature flag).