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/domain/relay/repository/label.rs

@@ -29,6 +29,15 @@ pub trait RelayLabelRepository: Send + Sync {
     /// Returns `RepositoryError::DatabaseError` if the database operation fails.
     async fn save_label(&self, id: RelayId, label: RelayLabel) -> Result<(), RepositoryError>;
 
+    /// Deletes the label for a specific relay.
+    ///
+    /// If no label exists for the relay, this operation succeeds without error.
+    ///
+    /// # Errors
+    ///
+    /// Returns `RepositoryError::DatabaseError` if the database operation fails.
+    async fn delete_label(&self, id: RelayId) -> Result<(), RepositoryError>;
+
     /// Retrieves all relay labels from the repository.
     ///
     /// Returns a vector of tuples containing relay IDs and their corresponding labels.