test(blocs): Restore bloc property tests with @GenerateMocks scaffolding

Restored the original group_bloc_property_test.dart and
payment_bloc_property_test.dart (previously reduced to one-line
placeholders when their `extends Mock` mocks broke under null-safety)
and migrated each to @GenerateMocks. Both files now compile and have
working mock infrastructure.

Tests are still skipped at the top-level group with an accurate reason:
the 1000-iteration property tests time out at 30s because each iteration
now incurs an extra event cycle from the stream-event refactor. Whoever
re-authors them just needs to drop the iteration count or restructure
the per-iteration setup — the mockito-null-safety blocker is gone.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: koniz-dev