Enhance Validation, Error Handling, and Request Builder

[DakshithaS]

Description

This subtask adds robust input validation using struct tags and validators, centralized error mapping for user-friendly messages, and a fluent RequestBuilder API for constructing HTTP requests with pre-flight checks, JSON bodies, and multipart data. It includes transactional repository operations for conflict prevention, enhanced OpenAPI generation with security schemes, and improved error constants. Challenges include ensuring validation covers all edge cases and maintaining performance in request building. Outcomes include more reliable and maintainable client code with better debugging and user feedback.