(PDB-722) Added missing validation for fields in vector forms for 'in' and 'extract' operators in API v4

Previously such validation was only implemented for if field is passed as a string.
So, I added a validation for both when it is passed as a string and when as a vector of strings.

Original problem described in PDB-722 seems to be already fixed, so that was the only related issue that we found.
Ah, I understand the confusion. I guess the missing context is that that's a fairly recent change -- previously only a string was allowed, so it may still be the more common use and users may not be aware of the change. Thanks for accomodating in any case.