Feature: Implement discounted bundles for product combinations

[Copilot]

Implements the requested bundle discount feature that allows customers to receive 10% off when purchasing all items in a product bundle.

What this PR does

This PR adds a new type of special offer - bundles - that provides discounts when customers buy complete sets of products together. When all items in a bundle are purchased, customers receive a 10% discount on the total price of those bundled items.

Key Features

• Complete bundle discounting: Only applies discount when all products in a bundle are purchased
• Partial bundle handling: No discount applied for incomplete bundles
• Multiple bundle support: Correctly handles multiple complete bundles in a single transaction
• Fractional quantity handling: Only whole items count toward bundle completion
• Configurable discount: Default 10% discount, but customizable percentage supported

Example Usage

const catalog = new FakeCatalog();
const toothbrush = new Product("toothbrush", ProductUnit.Each);
catalog.addProduct(toothbrush, 0.99);
const toothpaste = new Product("toothpaste", ProductUnit.Each);
catalog.addProduct(toothpaste, 1.79);

const teller = new Teller(catalog);
teller.addBundleOffer([toothbrush, toothpaste]); // 10% discount on €2.78

// Customer buys 1 toothbrush + 1 toothpaste = gets €0.28 discount
// Customer buys 2 toothbrushes + 1 toothpaste = gets €0.28 discount (only 1 complete bundle)

Implementation Details

The solution introduces minimal changes while maintaining backward compatibility:

• Added Bundle enum value to SpecialOfferType
• Created Bundle and BundleOffer classes for bundle data structures
• Extended Teller with addBundleOffer() method
• Added handleBundleOffers() method to ShoppingCart for discount calculation
• Comprehensive test suite covering all edge cases

Test Coverage

All existing tests continue to pass, plus 5 new test cases covering:

• Complete bundle discount calculation
• Partial bundle scenarios (no discount)
• Multiple complete bundles
• Excess items handling
• Fractional quantity edge cases

The implementation exactly matches the requirements in the original Dutch issue: "Wanneer je alle items in een productbundel koopt, krijg je 10% korting op het totaalbedrag voor die items."

Fixes #19

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