[MLv2] [BE] Make displayInfo for column groups straightforward to use

[ranquild]

Currently the FE has to use a utility function everywhere when displaying ColumnGroup:

export function getColumnGroupName(groupInfo: ColumnGroupDisplayInfo) {
  return groupInfo.fkReferenceName || groupInfo.displayName;
}

This is not straightforward. We never use fkReferenceName or its own, and we never use displayName for ColumnGroup outside of this function.

We should improve the API - only displayName should be used without additional checks on the FE. MLv2 should update displayName for ColumnGroup to be the result of this function under the hood.

Without this check when using raw displayName now:

With this check:

[snoe]

See #37347 here for a good reproduction to make column group displays more useful