feat(domain,presentation,tests): implement Relay entity, DTOs, and API errors

- Add Relay entity with constructors and business logic methods
- Add RelayDto for API responses with From<Relay> conversion
- Add ApiError with ResponseError trait for unified error handling
- Add dependency injection tests for mock infrastructure in test mode

backend/src/presentation/dto/mod.rs

@@ -0,0 +1,6 @@
+/// Relay-specific Data Transfer Objects.
+///
+/// This module contains DTO structures for relay-related API responses,
+/// providing serialized representations of relay domain objects for
+/// external consumption.
+pub mod relay_dto;

backend/src/presentation/dto/relay_dto.rs

@@ -0,0 +1,197 @@
+use poem_openapi::Object;
+use serde::{Deserialize, Serialize};
+
+use crate::domain::relay::Relay;
+
+/// Data Transfer Object for relay information.
+///
+/// This struct represents a relay in a serialized format suitable for API
+/// responses. It contains the relay's ID, current state, and label in a
+/// format that can be easily serialized to JSON.
+#[derive(Object, Serialize, Deserialize)]
+pub struct RelayDto {
+    /// The relay's unique identifier (1-8).
+    id: u8,
+    /// The relay's current state as a string ("on" or "off").
+    state: String,
+    /// The relay's user-friendly label.
+    label: String,
+}
+
+impl From<Relay> for RelayDto {
+    /// Converts a domain Relay object to a `RelayDto`.
+    ///
+    /// This conversion extracts the relay's ID, state, and label from the
+    /// domain object and formats them for API consumption.
+    ///
+    /// # Arguments
+    ///
+    /// * `value` - The Relay domain object to convert
+    ///
+    /// # Returns
+    ///
+    /// A `RelayDto` containing the relay's data in serialized format
+    fn from(value: Relay) -> Self {
+        let id = value.id().as_u8();
+        let state = value.state().to_string();
+        let label = value.label().to_string();
+        Self { id, state, label }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::domain::relay::types::{RelayId, RelayLabel, RelayState};
+
+    #[test]
+    fn test_relay_dto_from_relay_with_default_label() {
+        // Test: Relay with default label converts to RelayDto with None label
+        let relay_id = RelayId::new(1).unwrap();
+        let relay = Relay::new(relay_id);
+        let dto = RelayDto::from(relay);
+
+        assert_eq!(dto.id, 1);
+        assert_eq!(dto.state, "off");
+        assert_eq!(dto.label, "Unlabeled".to_string());
+    }
+
+    #[test]
+    fn test_relay_dto_from_relay_with_custom_label() {
+        // Test: Relay with custom label converts to RelayDto with Some(label)
+        let relay_id = RelayId::new(2).unwrap();
+        let label = RelayLabel::new("Water Pump".to_string()).unwrap();
+        let relay = Relay::with_label(relay_id, RelayState::On, label);
+        let dto = RelayDto::from(relay);
+
+        assert_eq!(dto.id, 2);
+        assert_eq!(dto.state, "on");
+        assert_eq!(dto.label, "Water Pump".to_string());
+    }
+
+    #[test]
+    fn test_relay_dto_from_relay_with_on_state() {
+        // Test: Relay with On state converts to RelayDto with "on" state
+        let relay_id = RelayId::new(3).unwrap();
+        let relay = Relay::with_state(relay_id, RelayState::On);
+        let dto = RelayDto::from(relay);
+
+        assert_eq!(dto.id, 3);
+        assert_eq!(dto.state, "on");
+        assert_eq!(dto.label, "Unlabeled".to_string());
+    }
+
+    #[test]
+    fn test_relay_dto_from_relay_with_off_state() {
+        // Test: Relay with Off state converts to RelayDto with "off" state
+        let relay_id = RelayId::new(4).unwrap();
+        let relay = Relay::with_state(relay_id, RelayState::Off);
+        let dto = RelayDto::from(relay);
+
+        assert_eq!(dto.id, 4);
+        assert_eq!(dto.state, "off");
+        assert_eq!(dto.label, "Unlabeled".to_string());
+    }
+
+    #[test]
+    fn test_relay_dto_from_relay_with_max_length_label() {
+        // Test: Relay with maximum length label (50 chars) converts correctly
+        let relay_id = RelayId::new(5).unwrap();
+        let max_label = RelayLabel::new("A".repeat(50)).unwrap();
+        let relay = Relay::with_label(relay_id, RelayState::Off, max_label);
+        let dto = RelayDto::from(relay);
+
+        assert_eq!(dto.id, 5);
+        assert_eq!(dto.state, "off");
+        assert_eq!(dto.label, "A".repeat(50));
+    }
+
+    #[test]
+    fn test_relay_dto_from_relay_with_empty_label_becomes_none() {
+        let relay_id = RelayId::new(6).unwrap();
+        let relay = Relay::new(relay_id);
+        let dto = RelayDto::from(relay);
+
+        assert_eq!(dto.id, 6);
+        assert_eq!(dto.state, "off");
+        assert_eq!(dto.label, "Unlabeled".to_string());
+    }
+
+    #[test]
+    fn test_relay_dto_serialization() {
+        // Test: RelayDto can be serialized to JSON
+        let relay_id = RelayId::new(7).unwrap();
+        let label = RelayLabel::new("Test Relay".to_string()).unwrap();
+        let relay = Relay::with_label(relay_id, RelayState::On, label);
+        let dto = RelayDto::from(relay);
+
+        let json = serde_json::to_string(&dto).unwrap();
+        assert_eq!(
+            json,
+            r#"{"id":7,"state":"on","label":"Test Relay"}"#
+        );
+    }
+
+    #[test]
+    fn test_relay_dto_deserialization() {
+        // Test: RelayDto can be deserialized from JSON
+        let json = r#"{"id":8,"state":"off","label":"Another Relay"}"#;
+        let dto: RelayDto = serde_json::from_str(json).unwrap();
+
+        assert_eq!(dto.id, 8);
+        assert_eq!(dto.state, "off");
+        assert_eq!(dto.label, "Another Relay".to_string());
+    }
+
+    #[test]
+    fn test_relay_dto_all_valid_relay_ids() {
+        // Test: All valid relay IDs (1-8) convert correctly
+        for id_val in 1..=8 {
+            let relay_id = RelayId::new(id_val).unwrap();
+            let relay = Relay::new(relay_id);
+            let dto = RelayDto::from(relay);
+
+            assert_eq!(dto.id, id_val);
+            assert_eq!(dto.state, "off");
+            assert_eq!(dto.label, "Unlabeled".to_string());
+        }
+    }
+
+    #[test]
+    fn test_relay_dto_state_toggle_reflected() {
+        // Test: Relay state changes are reflected in DTO
+        let relay_id = RelayId::new(1).unwrap();
+        let mut relay = Relay::with_state(relay_id, RelayState::Off);
+
+        // Initial state
+        let dto1 = RelayDto::from(relay.clone());
+        assert_eq!(dto1.state, "off");
+
+        // After toggle
+        relay.toggle();
+        let dto2 = RelayDto::from(relay.clone());
+        assert_eq!(dto2.state, "on");
+
+        // After another toggle
+        relay.toggle();
+        let dto3 = RelayDto::from(relay);
+        assert_eq!(dto3.state, "off");
+    }
+
+    #[test]
+    fn test_relay_dto_label_change_reflected() {
+        // Test: Relay label changes are reflected in DTO
+        let relay_id = RelayId::new(2).unwrap();
+        let mut relay = Relay::new(relay_id);
+
+        // Initial label (default)
+        let dto1 = RelayDto::from(relay.clone());
+        assert_eq!(dto1.label, "Unlabeled".to_string());
+
+        // After setting custom label
+        let new_label = RelayLabel::new("Custom Label".to_string()).unwrap();
+        relay.set_label(new_label);
+        let dto2 = RelayDto::from(relay);
+        assert_eq!(dto2.label, "Custom Label".to_string());
+    }
+}