[BE-SVC-020] be-svc: Implement request validation improvements
- Enhanced error messages in validator with more descriptive and contextual messages
- Added custom validations: slug, phone, date_iso, not_empty
- Created QueryParamValidation middleware for query parameter validation
- Support for validation rules: numeric, integer, min, max, oneof, email, uuid, url
- Improved error messages for all validation tags (40+ tags supported)
- Comprehensive unit tests for query parameter validation
- Better error context and user-friendly messages
Phase: PHASE-6
Priority: P2
Progress: 116/267 (43.45%)