feat: Remove unused imports and suppress dead_code warnings across charging engine modules

- Remove unused AsyncCommands/error/ChargingSession/ErrorContext imports from credit_management
- Remove unused ErrorContext/ChargingError imports from engine
- Remove unused ErrorContext import from rating_billing and prefix unused variables with underscore
- Remove unused ErrorContext import from rating_plans_repo
- Remove unused Context import from errors/helpers
- Remove unused info/debug/SystemTime imports from monitoring
- Remove unused sleep

apps/charging-engine/src/errors/helpers.rs

@@ -1,4 +1,3 @@
-use anyhow::Context;
 
 use super::types::ChargingError;
 

apps/charging-engine/src/errors/types.rs

@@ -10,9 +10,11 @@ pub enum ChargingError {
     SubscriberNotFound(String),
     RatingPlanNotFound(String),
     InsufficientCredit { available: u64, requested: u64 },
+    #[allow(dead_code)]
     UsageBlocked(String),
     InvalidInput(String),
     SerializationError(String),
+    #[allow(dead_code)]
     ConfigurationError(String),
     InternalError(String),
 }