feat: added discount coupon system

[adityacosmos24]

## PR: Discount Coupon System Implementation

Overview

This pull request adds a Discount Coupon System, showcasing the design and application of coupon-based discounts in an application. The solution demonstrates extensible object-oriented design, allowing for different types of coupons and discount strategies to be easily integrated.

Key Features

• Coupon Abstraction:
  Defines an abstract class or interface for coupons, ensuring a consistent contract for all discount policies.
• Multiple Coupon Types:
  Implements various coupon types (percentage-based, amount-based, etc.) to demonstrate the flexibility of the system.
• Strategy/Policy Pattern Usage:
  Applies relevant design patterns to encapsulate variable discounting logic.
• Composability:
  Designed such that adding new coupon types or modifying existing ones requires minimal future changes.
• End-to-End Flow:
  Provides main/test classes demonstrating how customers can apply, combine, or validate coupons during checkout or billing.

Code Structure

• Core Coupon Logic:
  • Coupon abstractions/interfaces (e.g., Coupon.h, Coupon.cpp)
  • Concrete implementations for each coupon type
• Demonstration/Test Files:
  • Main application file showing example scenarios
  • Test cases for different coupon applications

Design Principles Applied

• Open/Closed Principle—easy to add new coupon strategies without modifying core system logic.
• Single Responsibility Principle—separation of coupon logic from customer and billing logic.
• Encapsulation—each coupon’s logic is self-contained.

Review Checklist

• Code compiles and runs as expected
• Test/demo cases cover different coupon scenarios
• Well-documented and organized class structure
• New coupon types can be added with minimal changes

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closses #43