getDefaultFractionDigits() throws for currencies withdrawn from ISO 4217 (e.g. BGN, HRK)

Problem/Motivation

CurrencyRepository::getDefaultFractionDigits() reads the fraction digits from commerceguys/intl's base definitions, which follow the ISO 4217 list. When a currency is withdrawn from ISO 4217, the library drops it, and getDefaultFractionDigits() throws UnknownCurrencyException, even though the store still has the currency (and orders/payments in it).

Since payment gateways convert amounts through MinorUnitsConverter, refunding or capturing an existing payment in a withdrawn currency fails.

This has happened several times already:

• VEF (intl v1.0.2)
• HRK (intl v1.1.2, Croatia adopted the euro)
• SLL (intl v2.0.6)
• ANG, CUC, ZWL (intl v2.0.7)
• BGN (intl v2.0.8, Bulgaria adopted the euro)

Steps to reproduce

1. Import BGN with an older commerceguys/intl release (or have a store that already has it).
2. Update commerceguys/intl to v2.0.8 or later.
3. \Drupal::service('commerce_price.minor_units_converter')->toMinorUnits(new Price('10', 'BGN')); throws UnknownCurrencyException.

Proposed resolution

When the currency is not defined by the library, fall back to the fraction digits of the stored commerce_currency entity. CurrencyRepository::get() already throws UnknownCurrencyException when the entity doesn't exist either, so the method's contract doesn't change.

New stores still can't import withdrawn currencies, which is expected.

Remaining tasks

• MR with the fallback and a kernel test.