Add reusable test suite with 18 test functions covering get_label(),
save_label(), delete_label(), and get_all_labels() methods. Tests
verify contract compliance for any repository implementation.
Added delete_label() method to trait interface and implemented it in
MockRelayLabelRepository to support complete CRUD operations.
TDD phase: RED - Tests written before SQLite implementation (T036)
Ref: T035 (specs/001-modbus-relay-control/tasks.md)
