[ BigBrotr/bigbrotr ] test: update tests for refactored models and Brotr API
Update test suite to match the refactored architecture:
Model tests:
- test_event.py: Test __getattr__ delegation, from_db_params()
- test_keys.py: Test wrapper pattern instead of inheritance
- test_event_relay.py: Use Event wrapper, remove from_nostr_event tests
- test_relay.py: Test url_without_scheme key in _parse_url()
- test_relay_metadata.py: Update to_db_params() assertions