Extract hardcoded discount description strings to named constants

[Copilot]

Hardcoded discount description strings ("3 for 2", "% off") were scattered throughout shopping cart implementations across 14 languages, violating DRY principles and complicating future changes.

Changes

Extracted magic strings to named constants at the top of each ShoppingCart implementation:

Before:

discount = Discount(p, "3 for 2", -discount_amount)
discount = Discount(p, str(offer.argument) + "% off", -quantity * unit_price * offer.argument / 100.0)

After:

THREE_FOR_TWO_DESCRIPTION = "3 for 2"
PERCENT_OFF_SUFFIX = " % off"

discount = Discount(p, THREE_FOR_TWO_DESCRIPTION, -discount_amount)
discount = Discount(p, f"{offer.argument}{PERCENT_OFF_SUFFIX}", -quantity * unit_price * offer.argument / 100.0)

Implementations Updated

Applied language-appropriate constant patterns across:

• Python (2 variants): Module-level constants
• Java: private static final
• C# (2 variants): private const
• TypeScript: const
• Go: Package-level constants
• Ruby: Class constants
• Elixir: Module attributes (@)
• Common Lisp: defconstant
• Kotlin: Companion object constants
• PHP: Class constants
• Swift: Static properties
• C++: Static constants

Notes

• Preserved original output formatting (including " % off" with leading space)
• Dynamic strings ("2 for X", "5 for X") remain parameterized as they include variable amounts
• No behavioral changes

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