Original PR description

### Issue:
In 19.2, the EC invoice header was no longer rendered on invoices, falling back to the standard layout
The header includes legally required data and a barcode, both missing from the printed PDF

### Cause:
The XPath targeting `//t[@t-set='o']` relied on the position of the `t-set` for `o` in `report_invoice_document`
After the refactor in commit `eb6e88a25050`, the injection point no longer resolves as expected, preventing the header from rendering

Targeting `//div[hasclass('invoice_main')]` instead provides a stable anchor that is less sensitive to template restructuring

### Steps to reproduce:
- Install `l10n_ec_edi` and switch to the EC company
- Create and confirm an invoice
- Print the invoice

Before the fix, the EC header and barcode are missing from the PDF

opw-6375733