Implement missing fields for priceFromDraft function

[Copilot]

The priceFromDraft function in src/repositories/product/helpers.ts was missing several important fields when converting a PriceDraft to a Price object. This caused incomplete price objects to be created, missing key functionality like customer group targeting, validity periods, discounts, tiered pricing, and custom fields.

Changes Made

• customerGroup: Added mapping from CustomerGroupResourceIdentifier to CustomerGroupReference using the existing getReferenceFromResourceIdentifier helper
• validFrom/validUntil: Added direct string mapping for price validity date ranges
• discounted: Added mapping from DiscountedPriceDraft to DiscountedPrice with proper value conversion and ProductDiscount reference handling
• tiers: Added mapping from PriceTierDraft[] to PriceTier[] with value conversion for quantity-based pricing
• custom: Added mapping from CustomFieldsDraft to CustomFields using the existing createCustomFields helper

Testing

Added comprehensive unit tests in src/repositories/product/helpers.test.ts that verify:

• Basic price creation without optional fields continues to work
• CustomerGroup references are properly resolved and mapped
• ValidFrom/validUntil date strings are correctly passed through
• Discounted prices with ProductDiscount references work correctly
• Price tiers with quantity thresholds and values are properly converted
• Custom fields are correctly processed using the type system

All existing tests continue to pass, ensuring no regressions.

Example Usage

const priceDraft: PriceDraft = {
  value: { currencyCode: "EUR", centAmount: 1000 },
  country: "NL",
  customerGroup: { typeId: "customer-group", id: "vip-customers" },
  validFrom: "2024-01-01T00:00:00.000Z",
  validUntil: "2024-12-31T23:59:59.999Z",
  tiers: [
    { minimumQuantity: 10, value: { currencyCode: "EUR", centAmount: 900 } }
  ]
};

// Now correctly produces a complete Price object with all fields
const price = priceFromDraft(context, storage, priceDraft);

Fixes #283.

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