Fix parent link resolution and transmission KeyErrors

[piLeoni]

Summary

• Guard parent transforms when occurrenceTwo is missing; fallback to occurrence or base_link
• Ensure parent links are added to links_xyz_dict before transmission math
• Use safe defaults to avoid KeyErrors when computing parent-child offsets
• Improve error messages for missing parent links

Author

Pietro Leoni

Testing

• Ran exporter on assembly with B0/B1 parent-child; URDF generation now completes without KeyErrors.