APIv4 - Fix Activity contact virtual fields to work across joins

[colemanw]

Overview

This fixes an oversight that slipped by the review process in #25744

Before

The new virtual fields for Activity.source/target/assignee_contact_id worked well when they were attached to the main entity but not when part of a join.

After

Fixed to work in any part of the query. Test updated.

Technical Details

Setting the column name to id means the sqlRenderer callback function will receive the correct alias for that field depending on which part of the query it is used in (it's not always a.id).
While I was in there I updated the test and consolidated the two tests in that file which were basically testing the same functionality.
I was also able to get rid of the "formatOutputForMultipleActivityContactIds" helper function because the API can automatically explode comma-separated values when serialization method is declared for the field.

FYI @highfalutin

[civibot]

(Standard links)

• If you are reviewing this pull-request, you may wish to consult the test sites and the Review Standards (long template, short template).

[eileenmcnaughton]

Merging based on the unit test cover - this is really well tested code & the additional functionality is tested