Feature: Add discounted bundles support to supermarket receipt system

[Copilot]

This PR implements the new discounted bundles feature requested in the issue. When customers buy all items in a product bundle, they receive a 10% discount on the total for those items.

Key Features

Bundle Discount Logic:

• Only complete bundles receive discounts
• 10% discount applied to the total price of bundle items
• Multiple complete bundles are all discounted
• Extra items beyond complete bundles are charged at regular price

Example Usage:

// Create a bundle offer for toothbrush + toothpaste
List<Product> bundleProducts = Arrays.asList(toothbrush, toothpaste);
teller.addBundleOffer(bundleProducts, 10.0);

// Customer buys 1 toothbrush (€0.99) + 1 toothpaste (€1.79)
// Gets 10% discount: €2.78 - €0.278 = €2.502 total

Implementation Details:

• Added BUNDLE to SpecialOfferType enum
• Created BundleOffer class to represent bundle offers
• Extended Teller class with addBundleOffer() method
• Added handleBundleOffers() method to ShoppingCart for discount calculation
• Bundle discounts work alongside existing product-specific offers

Test Coverage:

• Basic bundle discount functionality
• Partial bundle handling (no discount for incomplete bundles)
• Multiple complete bundles
• Integration with existing offer types

The implementation follows the existing codebase patterns and maintains backward compatibility with all existing functionality.

Fixes #8.

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