Refactor Product Controller for enhanced structure and error handling.

[syedwajahat01]

PR Description:
This PR introduces key improvements to the ProductController, focusing on cleaner code structure, modern Spring Boot practices, and robust error handling.

Changes Implemented:

• Refactored Request Mappings:
  - Replaced @RequestMapping with more specific annotations (@GetMapping and @PostMapping) for better readability
  and HTTP method clarity.
  - Introduced a base path (/product) for all endpoints to reduce redundancy and improve URL structure.

• Enhanced Error Handling:

      - Implemented comprehensive error handling using ResponseEntity to return appropriate HTTP status codes (200 OK, 
         201 Created, 404 Not Found, 500 Internal Server Error).
      - Added a private method getProductById to centralize prouct retrieval and error response, reducing code duplication. 
  

• Improved Dependency Injection:
  - Switched from setter-based to constructor-based dependency injection for the ProductService, aligning with best
  practices for immutability and testability.

• Optimized Code Readability:
  - Removed unnecessary System.out.println statements to clean up the code.
  - Structured methods to ensure single responsibilities, enhancing maintainability.

Benefits:
- Cleaner and More Maintainable Code: The refactoring enhances code clarity, making future updates and debugging easier.
- Better API Responses: Clients now receive meaningful HTTP responses, improving error handling and overall user experience.
- Alignment with Best Practices: The updates bring the controller in line with modern Spring Boot conventions, ensuring the
codebase is up-to-date and professional.