Get the system default payment provider

Response shape for GET /billing/payment-provider/ and its unauthenticated /default/ sibling: the resolved provider slug plus that provider's public credentials. Only the object matching provider is populated; the other is null, so a client never receives keys for a provider it did not resolve to.

Plain serializers.Serializer -- nothing here is DB-backed. Serializes a vinta_billing.services.provider_credentials.PublicProviderCredentials instance, not a model, hence the explicit to_representation rather than attribute-matching nested serializers (the dataclass's flat stripe_publishable_key/mercadopago_public_key fields don't line up 1:1 with this serializer's nested stripe/mercadopago objects).