Fix undercoverage in GH importer

Summary

Recent merge requests for the GitHub importer have encountered undercoverage errors that need to be addressed.

Related Merge Requests

• !214563 (merged) - "Make 4XX responses not retriable for attachment downloads" - undercoverage failure mentioned by @georgekoltsov
• !215079 (merged) - "Improve handling of attachment urls and filenames" - undercoverage failure

Both MRs required the pipeline:skip-undercoverage label to proceed with merging.

Action Items

• Review the undercoverage failures in both MRs
• Add appropriate test coverage for the changes introduced
• Ensure future GitHub importer changes maintain adequate test coverage

References

• !214563 (merged)
• !215079 (merged)