Adds test coverage for the new GitLabIssueRef type used by the
MR closes_issues API endpoint:
- deserializes_gitlab_issue_ref: Single object with all fields
- deserializes_gitlab_issue_ref_array: Array of refs (typical API response)
Validates that cross-project references (different project_id values)
deserialize correctly, which is important for cross-project close links.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>