`fields_have_appropriate_selections` does not require selections on union types

[gmac]

Describe the bug

The fields_have_appropriate_selections validation presently allows the following selection:

query { myUnion }

With myUnion being a union type, this query returns the following payload:

{ "data": { "myUnion": {} } }

The spec would require a union type to make a selection.

Cause

The issue occurs here: https://github.com/rmosolgo/graphql-ruby/blob/master/lib/graphql/static_validation/rules/fields_have_appropriate_selections.rb#L34

The cause of the issue is that the fields? check omits union types. To fix this validation, the check should use composite? instead of fields?.

Steps to resolution

Unfortunately this is a breaking change as sources that used to validate will now fail. Need to:

• Fix the relevant check.
• Hook breaking behavior to a schema-level setting for spec-compliance. Should also include https://github.com/rmosolgo/graphql-ruby/issues/4403