refactor: Reorganize charging and errors modules into proper directory structure

- Move charging module files into charging/ directory (types.rs, engine.rs, credit_management.rs, rating_billing.rs)
- Move error module files into errors/ directory (types.rs, helpers.rs)
- Create mod.rs files for charging and errors modules with simplified exports
- Update import paths to use super:: for intra-module references
- Remove charging_new/errors_new aliases and use charging/errors directly
- Simplify module

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

@@ -0,0 +1,2 @@
+pub mod types;
+pub mod helpers;