test(settings): write tests for CorsSettings struct deserialization

Add 5 comprehensive tests for CorsSettings in TDD red phase:
- cors_settings_deserialize_from_yaml: YAML deserialization
- cors_settings_default_has_empty_origins: Default values
- cors_settings_with_wildcard_deserializes: Wildcard origin handling
- settings_loads_cors_section_from_yaml: Settings integration
- cors_settings_deserialize_with_defaults: Partial deserialization

Tests are intentionally failing (red phase) as CorsSettings struct
will be implemented in T010. Follows TDD requirement to write tests
before implementation.

Ref: T009 (specs/001-modbus-relay-control/tasks.md)

backend/src/settings.rs

@@ -336,4 +336,115 @@ mod tests {
         assert_eq!(settings.burst_size, 100); // default
         assert_eq!(settings.per_seconds, 60); // default
     }
+
+    // T009: Tests for CorsSettings struct (TDD - write tests first)
+    #[test]
+    fn cors_settings_deserialize_from_yaml() {
+        let yaml = r#"
+            allowed_origins:
+              - "http://localhost:5173"
+              - "https://sta.example.com"
+            allow_credentials: true
+            max_age_secs: 7200
+        "#;
+        let settings: CorsSettings = serde_yaml::from_str(yaml).unwrap();
+        assert_eq!(settings.allowed_origins.len(), 2);
+        assert_eq!(settings.allowed_origins[0], "http://localhost:5173");
+        assert_eq!(settings.allowed_origins[1], "https://sta.example.com");
+        assert!(settings.allow_credentials);
+        assert_eq!(settings.max_age_secs, 7200);
+    }
+
+    #[test]
+    fn cors_settings_default_has_empty_origins() {
+        let settings = CorsSettings::default();
+        assert!(
+            settings.allowed_origins.is_empty(),
+            "Default CorsSettings should have empty allowed_origins for restrictive fail-safe"
+        );
+        assert!(
+            !settings.allow_credentials,
+            "Default CorsSettings should have credentials disabled"
+        );
+        assert_eq!(
+            settings.max_age_secs, 3600,
+            "Default CorsSettings should have 1 hour max_age"
+        );
+    }
+
+    #[test]
+    fn cors_settings_with_wildcard_deserializes() {
+        let yaml = r#"
+            allowed_origins:
+              - "*"
+            allow_credentials: false
+            max_age_secs: 3600
+        "#;
+        let settings: CorsSettings = serde_yaml::from_str(yaml).unwrap();
+        assert_eq!(settings.allowed_origins.len(), 1);
+        assert_eq!(settings.allowed_origins[0], "*");
+        assert!(!settings.allow_credentials);
+        assert_eq!(settings.max_age_secs, 3600);
+    }
+
+    #[test]
+    fn settings_loads_cors_section_from_yaml() {
+        // Create a temporary settings file with CORS configuration
+        let yaml_content = r#"
+application:
+  name: "test-app"
+  version: "1.0.0"
+  port: 3100
+  host: "127.0.0.1"
+  base_url: "http://127.0.0.1:3100"
+  protocol: "http"
+
+debug: false
+frontend_url: "http://localhost:5173"
+
+rate_limit:
+  enabled: true
+  burst_size: 100
+  per_seconds: 60
+
+cors:
+  allowed_origins:
+    - "http://localhost:5173"
+  allow_credentials: false
+  max_age_secs: 3600
+
+modbus:
+  host: "192.168.0.200"
+  port: 502
+  slave_id: 0
+  timeout_secs: 5
+
+relay:
+  label_max_length: 50
+        "#;
+
+        // Use serde_yaml to deserialize directly
+        let settings: Settings = serde_yaml::from_str(yaml_content).unwrap();
+
+        // Verify CORS settings were loaded
+        assert_eq!(settings.cors.allowed_origins.len(), 1);
+        assert_eq!(settings.cors.allowed_origins[0], "http://localhost:5173");
+        assert!(!settings.cors.allow_credentials);
+        assert_eq!(settings.cors.max_age_secs, 3600);
+    }
+
+    #[test]
+    fn cors_settings_deserialize_with_defaults() {
+        // Test partial deserialization using serde defaults
+        let yaml = r#"
+            allowed_origins:
+              - "https://example.com"
+        "#;
+        let settings: CorsSettings = serde_yaml::from_str(yaml).unwrap();
+        assert_eq!(settings.allowed_origins.len(), 1);
+        assert_eq!(settings.allowed_origins[0], "https://example.com");
+        // These should use defaults
+        assert!(!settings.allow_credentials);
+        assert_eq!(settings.max_age_secs, 3600);
+    }
 }