change to return the actual nodes that have loop

[adriantam]

Description

Update node detection to return list of nodes that have loop. This allows caller to return the proper error message.

References

Review Checklist

• [ ] I have clicked on "allow edits by maintainers".
• [ ] I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• [ ] The correct base branch is being used, if not main
• [ ] I have added tests to validate that the change in functionality is working as expected