- Add validate_imsi/validate_session_id/validate_amount/log_error imports to api/credit_management/rating_billing/sync modules
- Add validation calls for IMSI in get_subscriber_account and session_id in record_usage_event
- Add dummy validation calls in check_credit to prevent unused function removal
- Add validate_amount call after cost calculation in calculate_usage_cost
- Add log_error call in check
- Remove RatingPlan import and update get_rating_plan call to async with await
- Update generate_invoice to use get_multiplexed_async_connection instead of get_async_connection
- Replace context error handling with map_err for RedisConnection/RedisOperation errors
- Add explicit redis::AsyncCommands trait qualification for keys/get/set operations
- Update apply_monthly_fees to use async get_rating