Approve one or more invoices

This endpoint allows bulk approval of invoices. Each invoice is processed independently,
enabling partial success scenarios where some invoices may be approved while others fail validation.

Conditions:

• Only invoices with status 'Draft' or 'Pending Approval' can be approved.

Effects of approval:

• Invoice status is updated to 'Approved'.
• All related Invoice Line Items are approved.
• Related Billing Schedule Records and Billing Schedule Details are marked as 'Invoiced'.
• Billing Header Rollup - Total Invoice Amount and Total Pending Amount are updated.