[17.0][FIX] account_invoice_facturx: fix Factur-X 4.x schematron errors

[tendil]

With factur-x 4.x, schematron validation became stricter and the module
started generating EN16931 XML that no longer passed validation for
credit transfer invoices.

Two concrete problems caused that failure:

• the seller contact email was exported with a schemeID attribute on
  URIID, although this attribute is not allowed in that context;
• when the creditor account was not an IBAN, the module did not export
  a fallback account identifier, so the payment means block failed
  BR-CO-27.

As a result, invoices that were previously generated successfully with
factur-x 3.x started failing during Factur-X XML generation with
factur-x 4.x.

---

This patch fixes the XML generation logic by:

• removing the invalid schemeID attribute from the seller email URIID;
• exporting ProprietaryID for non-IBAN creditor accounts;
• raising a clear UserError when no account identifier is available,
  instead of generating XML that is known to be invalid.

The test suite is updated accordingly:

• the default invoice fixture now includes a valid creditor account;
• a regression test checks that URIID is generated without schemeID;
• a regression test checks that non-IBAN accounts are exported as
  ProprietaryID;
• a regression test checks that missing account identifiers are rejected
  explicitly.

Task: 5347

[OCA-git-bot]

Hi @alexis-via,
some modules you are maintaining are being modified, check this out!