feat: Add list_rating_plans error handling, dead_code attributes, and refactor monitoring/models

- Add error mapping to list_rating_plans in engine.rs
- Add dead_code attributes to get/list_map/deactivate methods in rating_plans_repo and row_to_plan function
- Add list method to RatingPlansRepo returning Vec<RatingPlan>
- Add dead_code attribute to validate_imsi in helpers
- Add UsageBlocked and ConfigurationError variants to ChargingError with Display/IntoResponse implementations
- Add Default

apps/charging-engine/src/models.rs

@@ -43,3 +43,12 @@ pub struct HealthResponse {
     pub status: String,
     pub timestamp: chrono::DateTime<chrono::Utc>,
 }
+
+impl Default for HealthResponse {
+    fn default() -> Self {
+        Self {
+            status: "OK".to_string(),
+            timestamp: chrono::Utc::now(),
+        }
+    }
+}