test(domain/relay): write failing tests for RelayState serialization

Tests verify serialization and deserialization of RelayState enum with
"on" and "off" states. Red phase of TDD - tests define expected behavior
before implementation.

Ref: T019 (specs/001-modbus-relay-control/tasks.md)

backend/src/domain/relay/types/mod.rs

@@ -1,2 +1,5 @@
 mod relayid;
+mod relaystate;
+
 pub use relayid::RelayId;
+pub use relaystate::RelayState;

backend/src/domain/relay/types/relaystate.rs

@@ -0,0 +1,52 @@
+/// State of a relay (on or off).
+///
+/// Represents the binary state of a relay channel in the Modbus controller.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum RelayState {
+    /// Relay is energized (circuit closed).
+    On,
+    /// Relay is de-energized (circuit open).
+    Off,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_relay_state_on_serializes_to_on() {
+        // Test: RelayState::On → serializes to "on"
+        let state = RelayState::On;
+        let json = serde_json::to_string(&state).unwrap();
+        assert_eq!(json, r#""on""#);
+    }
+
+    #[test]
+    fn test_relay_state_off_serializes_to_off() {
+        // Test: RelayState::Off → serializes to "off"
+        let state = RelayState::Off;
+        let json = serde_json::to_string(&state).unwrap();
+        assert_eq!(json, r#""off""#);
+    }
+
+    #[test]
+    fn test_parse_on_from_string() {
+        // Test: Parse "on" from string
+        let state: RelayState = serde_json::from_str(r#""on""#).unwrap();
+        assert_eq!(state, RelayState::On);
+    }
+
+    #[test]
+    fn test_parse_off_from_string() {
+        // Test: Parse "off" from string
+        let state: RelayState = serde_json::from_str(r#""off""#).unwrap();
+        assert_eq!(state, RelayState::Off);
+    }
+
+    #[test]
+    fn test_parse_invalid_string_fails() {
+        // Test: Parse invalid string fails
+        let result: Result<RelayState, _> = serde_json::from_str(r#""invalid""#);
+        assert!(result.is_err());
+    }
+}