Deduplicate the copy-pasted update() method on the DetailsAttribute classes

[TimEvci]

Summary

The update() method is copy-pasted onto each *DetailsAttribute class, and it is identical to MoneybirdModel.update(). That's four copies of the same non-obvious logic.

Details

The same body:

def update(self, data: dict[str, Any]) -> None:
    validated = self.model_validate({**self.model_dump(), **data})
    for key in self.__class__.model_fields:
        object.__setattr__(self, key, getattr(validated, key))

appears in:

• MoneybirdModel.update (moneysnake/model.py:36)
• SalesInvoiceDetailsAttribute.update (moneysnake/sales_invoice.py:26)
• ExternalSalesInvoiceDetailsAttribute.update (moneysnake/external_sales_invoice.py:24)
• DocumentDetailsAttribute.update (moneysnake/document.py:32)

The logic is subtle (it uses model_validate plus object.__setattr__ to run field validators without flattening nested models to dicts), so having four copies is a real maintenance risk.

Suggested fix

Extract a thin base holding update() (and to_dict()), and have both MoneybirdModel and the *DetailsAttribute classes inherit it, e.g.:

class UpdatableModel(BaseModel):
    def to_dict(self) -> dict[str, Any]:
        return self.model_dump(exclude_none=True)

    def update(self, data: dict[str, Any]) -> None:
        ...

Acceptance criteria

• update() is defined once.
• The *DetailsAttribute classes behave identically to today.
• Existing detail-update tests pass unchanged.