Check a draft invoice against the e-invoicing conformance rules

Submits the generated e-invoice (Factur-X / CII) to the e-invoicing platform's validator and returns the findings. The invoice is NOT finalized and NOT transmitted — call this before Finalize to find out whether it would pass. Draft invoices only: a finalized invoice returns 409. The check re-renders the e-invoice from current data, which is not the document archived and transmitted at finalization, so the answer would not mean what it appears to mean. Read isValid for the verdict: an invoice can be non-conformant while every finding is only a warning, because the French BR-FR rules are reported as warnings.