IAV-252/feat(custom-instructions): Add FAQ category support and refactor service layer

Summary

This MR introduces FAQ category association for custom instructions and refactors the service layer for improved consistency and maintainability.

Changes

New Features

• FAQ Category Support: Custom instructions can now be linked to FAQ categories
  • Required for SUGGESTIONS category instructions
  • Forbidden for RESTRICTIONS category instructions
• Bulk Deactivation: New deactivateInstructionsByFaqCategory() method allows deactivating all instructions associated with a specific FAQ category (useful when a FAQ category is deleted or disabled)

Refactoring

• Unified agent lookup using AgentValidationHelper.findAgentByUidOrThrow() across all methods
• Extracted getAgentCustomInstructionByUidOrThrow() helper to eliminate duplicated instruction lookup code
• Added resolveFaqCategory() helper for centralized FAQ category validation logic
• Removed unused CustomerCareAgentRepository dependency

DTOs & Mapper Updates

• Added faqCategoryUid to request DTO
• Added faqCategoryUid and faqCategoryName to response DTO
• Updated mapper to use ModelMapper with manual field overrides for related entities

New Validations

Error Code	Condition
FAQ_CATEGORY_REQUIRED	SUGGESTIONS instruction without FAQ category
FAQ_CATEGORY_NOT_ALLOWED	RESTRICTIONS instruction with FAQ category
FAQ_CATEGORY_NOT_FOUND	Provided FAQ category UID doesn't exist

Testing

• Create SUGGESTIONS instruction with valid FAQ category ✅
• Create SUGGESTIONS instruction without FAQ category → Error ✅
• Create RESTRICTIONS instruction without FAQ category ✅
• Create RESTRICTIONS instruction with FAQ category → Error ✅
• Update instruction with FAQ category change ✅
• Bulk deactivate instructions by FAQ category ✅

Related Issues

Closes IAV-252