test(infrastructure): write RelayLabelRepository trait tests

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)

backend/src/infrastructure/persistence/mod.rs

@@ -6,5 +6,9 @@
 /// Mock repository implementation for testing.
 pub mod label_repository;
 
+/// Comprehensive tests for `RelayLabelRepository` trait contract (T035).
+#[cfg(test)]
+pub mod label_repository_tests;
+
 /// `SQLite` repository implementation for relay labels.
 pub mod sqlite_repository;