ES|QL: tests for FORK's evaluation of field names used in field_caps resolve calls

[astefan]

Description

Following the improvements made to FORK, it is necessary for the field names used in field_caps calls the IndexResolver is performing to be correctly tested and covered for fork as well.

These tests should go in IndexResolverFieldNamesTests.
Also, a comment here would be appreciated. The fieldNames method is trying to avoid asking for all fields from field_caps for performance reasons and it's trying to be smart in what fields it actually needs. It would help code maintainers and contributors to understand why fork needs all the fields in all cases.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[astefan]

@ioanatia I am assigning this one to you, since you were the main contributor to this area regarding the fork functionality. Please, reassign accordingly if this should be better handled by another contributor. Thank you.

[ioanatia]

I added this as a sub task for #121950.
We initially fixed this with #126074, but then as you noticed we ended up asking for all fields again with #125937.

[astefan]

Since #128501 reverted the previous changes made to fields used in _field_caps call relate d to fork, I am reopening this issue to keep this improvements aspect on the radar.