FieldValidation should be able to run agents

@Marcus - you've put the ticket to needs review but the MR is in draft: is this ready for review? Can you also explain the reasoning behind this being added here rather than in the AI module's ai_validation sub-module? Given the dependency on another module that most sites won't be using, I would have thought this would have made more sense as its own separate contrib module if we're thinking of deprecating the ai_validations sub-module?