test: add backup module tests
Adds 10 tests covering:
- backup_decoder_full / minimal / missing_required (decoder unit tests)
- backup_create_ok / with_ttl / api_error
- backup_list_ok / empty / api_error
- backup_restore_ok / api_error
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>