IAV-691/Enhance exception handling with centralized error codes

Summary

This MR introduces a centralized business error code system for the iavia-agent-service-customer-care microservice, improving traceability, consistency, and maintainability of exception handling.

Changes

New Components

Component	Description
BusinessErrorCode enum	Centralized error codes following CUSTOMER-CARE-SERVICE-NNN pattern
BusinessErrorCodeTest	Unit tests ensuring code uniqueness, range compliance, and message quality

Refactored Layers

• Helpers: AgentCreationHelper, AgentValidationHelper, UserHelper, WebhookHelper
• Inter-service communication: FileManagerServiceImpl, N8nServiceImpl, UserServiceImpl
• Business services: All service implementations updated

Alignment

This implementation aligns with the exception management improvements made in mc-starter.

Testing

• ✅ All error codes validated for uniqueness
• ✅ Code format validated against pattern CUSTOMER-CARE-SERVICE-\d{3}$
• ✅ Code numbers validated within assigned category ranges
• ✅ Messages validated for quality (uniqueness, formatting, minimum length)

Related

Closes IAV-691