src/checkout/ holds code without the PayPal SDK or Strapi at runtime, so it can be
unit tested without node_modules:
- vat.ts: vatIncludedDecimal and vatDecimal. config/constants.ts re-exports them
under the same names and keeps vatDecimalExcluded.
- pricing.ts: calculateTotalProductPrice, unchanged. The product service
re-exports it, so every importer keeps working.
- paypal-order.ts: the order request createSessionOrThrow builds, in two halves.
buildPayPalOrderItems (the total guard and the unsent items) still runs
before the try block, and buildPayPalOrderRequest inside it. Errors and log
lines stay as they were, for every input. The SDK enums are replaced by their
string values (CAPTURE, SET_PROVIDED_ADDRESS).
No behaviour change.
Refs libretech/mp#71
