Global VAT exemption reasons

Added support for providing VAT exemption reasons at document level, used when calculating VAT subtotals.

We’ve improved how VAT exemption reasons are modeled and validated.

Document-level exemption info: The send invoice and credit note schemas now support a vat object that can either contain full VAT subtotals or just a global exemptionReason / exemptionReasonCode.
Automatic VAT calculation: When only the exemption fields are provided, Recommand calculates VAT subtotals from the lines and applies the same exemption reason to all exempt categories.
Validation-aware behaviour: For documents with validation enforcement enabled, required exemption reasons are checked before generating UBL, so missing reasons surface as clear errors instead of only appearing in downstream validators.

This makes it easier to send compliant VAT-exempt invoices and credit notes without having to manually build detailed VAT subtotal structures.