[Security Solution][Detection Engine] custom highlighted fields not displaying in preview flyout

[vitaliidm]

Describe the bug:

Custom highlighted fields not displaying in preview flyout

Kibana/Elasticsearch Stack version:

At least 8.14, but likely early versions affected too

Steps to reproduce:

1. Add custom highlighted field for detection rule
2. Run rule preview
3. Observe custom highlighted field is not displayed in Highlighted fields of Investigation section
4. Same custom highlighted field is displayed, when opening alert preview of actually generated alert.

Screenshots (if relevant):

Preview flyout

Actual alert flyout

Any additional context (logs, chat logs, magical formulas, etc.):

This happens because highlighted fields component fetches rule and takes highlighted fields property from it. ruleId used to fetch rule is not the id of the rule. So, fetched rule is always undefined, thus custom highlighted fields are always empty. Even if id is correct in this context, not saved in rule custom highlighted fields won't be displayed correctly if fetched from backend. Probably, in this case, rule information should be taken from the form itself or from saved in alert rule.

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)

[elasticmachine]

Pinging @elastic/security-detections-response (Team:Detections and Resp)

[elasticmachine]

Pinging @elastic/security-detection-engine (Team:Detection Engine)

[vitaliidm]

cc: @yctercero

[yctercero]

I think we can wrap this into https://github.com/elastic/kibana/issues/171057 - great catch!