Add missing addParcelToDelivery order update action

[Copilot]

This PR implements the missing addParcelToDelivery order update action in the commercetools mock library.

Changes Made

Implementation

• Added OrderAddParcelToDeliveryAction import and Parcel type to order actions
• Implemented addParcelToDelivery method in OrderUpdateHandler class
• Added support for all optional parcel properties:
  • parcelKey - User-defined unique identifier
  • measurements - Parcel dimensions and weight
  • trackingData - Shipment tracking information
  • items - Line items or custom line items in the parcel
  • custom - Custom fields for the parcel

Features

• Supports identification of target delivery by either deliveryId or deliveryKey
• Proper validation with clear error messages for invalid inputs
• Generates proper parcel objects with base resource properties (id, version, createdAt, etc.)
• Integrates with existing custom fields handling via createCustomFields helper

Testing

• Added comprehensive test cases covering successful scenarios with both deliveryId and deliveryKey
• Tests all parcel properties (measurements, tracking data, items)
• Includes error handling tests for missing delivery identifiers and non-existent deliveries
• All existing tests continue to pass, ensuring no regressions

Example Usage

// Add a parcel to a delivery by ID
{
  action: "addParcelToDelivery",
  deliveryId: "delivery-123",
  parcelKey: "parcel-001",
  measurements: {
    heightInMillimeter: 100,
    lengthInMillimeter: 200,
    widthInMillimeter: 150,
    weightInGram: 500
  },
  trackingData: {
    trackingId: "TRACK123",
    carrier: "DHL"
  },
  items: [
    {
      id: "line-item-1",
      quantity: 1
    }
  ]
}

Fixes #284.

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