Read Customer by Reference

Updates the customer. ## Tax and Business Identifiers Send `entity_identifier_kind` and `entity_identifier_value` together to store the customer's tax or business identifier, such as an EU VAT number, a French SIREN, or a LEI. A customer holds one identifier at a time, so saving an identifier of a different kind replaces the existing one. The `vat_eu` and `national_tax` kinds also require `vat_country`. An unsupported kind, a missing or mismatched `vat_country`, or a `gln`, `duns`, or `lei` value in the wrong format returns `422`. Always send the kind. `entity_identifier_value` on its own is stored as a `company_reg` when no `vat_country` is present, and returns `422` naming `entity_identifier_kind` when one is. To clear an identifier, send a supported `entity_identifier_kind` with a blank `entity_identifier_value`, or send a blank `vat_number` on its own. The first form also clears `vat_number` and `vat_country`, and it removes whichever identifier the customer holds, whatever kind you send with it. The legacy `vat_number` and `vat_country` pair still works on its own. When neither entity identifier field is sent, Advanced Billing derives the kind from `vat_country`: an EU member state code or `GB` gives `vat_eu`, one of the national tax country codes gives `national_tax`, and a blank or unrecognized country gives `company_reg`. Sending a customer response straight back leaves the tax ID alone. A blank pair, and a pair that still matches the stored identifier with `vat_country` unchanged, are read as nothing to change rather than as a request to clear. For `gln`, `duns`, and `lei` that also covers the `vat_number` the response mirrors back, so the kind survives the round trip. What you do change is applied, and the entity identifier fields take precedence over `vat_number`. A different kind or value writes that identifier, and `vat_number` and `vat_country` follow from it. A different `vat_country` next to an unchanged pair is a real edit, so it is validated and can return `422`. Changing only `vat_number` leaves the pair unchanged, so the derivation above decides the kind, which turns a `gln`, `duns`, or `lei` customer into a `company_reg`. Setting `vat_number` to `null` or a blank string still clears the identifier. The response reports the stored identifier in `entity_identifier_kind` and `entity_identifier_value`, and repeats its value in `vat_number`.