(Custom) validation should work in stateless forms, and prevent user from submitting via `InstantiationButton`

When in a stateless form, the backend never stores anything, and thus backend-based validation does not work. However, we have validation support using expressions (and via JsonSchema, and `required`-validation), so we should still make sure to run those validations and prevent the user from "submitting" the stateless form via `InstantiationButton` if the form does not validate correctly.

### Relevant links
- https://altinn.slack.com/archives/CCQEQKGJD/p1705408256631589

Start by tracing stateless form handling and the InstantiationButton submission path, then compare how expression, JsonSchema, and required validation are applied. Done means those validations run for stateless forms and an invalid form cannot be submitted through InstantiationButton.

Written by the indexing model from the issue text.