Customer invoices - Adding `product_id` support in e-invoice import options

We're extending the Customer E-Invoice import endpoint to accept a product_id per invoice line in the invoice_options payload.

Impacted endpoint:

• Import a Customer E-Invoice:
  • POST /api/external/v2/customer_invoices/e_invoices/imports
  • scope: customer_invoices:all

The invoice_lines array in invoice_options now accepts an additional optional field:

• product_id: the product ID to assign to that line.

Updated invoice_options structure:

{
  "customer_id": 123,
  "invoice_lines": [
    {
      "e_invoice_line_id": "1",
      "ledger_account_id": 456,
      "product_id": 789
    }
  ]
}

This is a non-breaking change, existing integrations will continue to work without modification. The product_id field is optional and does not affect the existing request structure.