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.