VERI*FACTU's op-class should take precedence over regime

[methodofaction]

Assuming a structure like this:

                        "rates": [
                            {
                                "key": "reverse-charge",
                                "ext": {
                                    "es-verifactu-regime": "01",
                                    "es-verifactu-op-class": "S2"
                                },
                                "base": "10.00",
                                "amount": "0.00"
                            }
                        ],
                        

… pdf

Our PDF will render "01" as the exemption reason (in reality it's random but placing it first makes it more likely to happen).

We need to give priority to es-verifactu-op-class.

Related support ticket:
https://app.usepylon.com/issues?conversationID=a8e085b4-0c4c-4c7c-94be-aee339a6ff63