Enforce require-selections on FragmentSpreads within GraphQLUnionTypes

[maciesielka]

Description

Enforces require-selections on FragmentSpreads within GraphQLUnionTypes, like it already does for InlineFragments

Fixes #2318

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

(Technically this can cause new violations in existing codebases, but I'd consider this to be missing functionality and thus a bug more than a new feature).

How Has This Been Tested?

Added a unit test that describes and reproduces the issue well.

Checklist:

• I have followed the
  CONTRIBUTING doc and the
  style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests and linter rules pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[changeset-bot]

🦋 Changeset detected

Latest commit: 3c5868f

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@graphql-eslint/eslint-plugin	Major

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[maciesielka]

@dimaMachina any guidance for how to get this reviewed?