Remove redundant list-coercion validators (pydantic v2 already coerces list items)

[TimEvci]

Summary

Every model defines a @field_validator (ensure_payments, ensure_details, ensure_contact_people, ensure_financial_mutations) delegating to the ensure_list_of helper (model.py:10). These validators are fully redundant and can be deleted with zero behavior change — verified empirically, details below.

Details (verified)

Two facts make the validators dead weight:

1. Pydantic v2 coerces natively. For a field typed list[Payment], pydantic constructs Payment from a dict and preserves already-constructed instances — before any @field_validator runs. Proof in-repo: ExternalSalesInvoice.details has no validator and coerces dicts correctly today.

2. The None guards never execute on the non-optional fields. @field_validator defaults to mode="after", which runs after type validation. For non-optional fields (SalesInvoice.payments/details, Document.payments/details, FinancialStatement.financial_mutations), a None input fails the list[...] type check before the validator is ever called. Verified: SalesInvoice(payments=None) and FinancialStatement(financial_mutations=None) raise ValidationError today — the if value is None: return [] branches are unreachable.

For the optional fields (Contact.contact_people, ExternalSalesInvoice.payments, both typed list[...] | None), the validator receives None and returns None — identical to having no validator at all.

So in every case, deleting the validator changes nothing.

⚠️ Do NOT normalize None to [] as part of this change

An earlier draft suggested a BeforeValidator(lambda v: v or []). That is not behavior-preserving:

• For Contact.contact_people and ExternalSalesInvoice.payments, it would turn None into [] (today None is preserved).
• For the non-optional fields, it would make a null-from-API start succeeding where it currently raises ValidationError.

Both may be desirable, but they are behavior changes and belong to a separate decision (see below), not this cleanup.

Suggested fix (scoped)

1. Delete all ensure_* field validators listed above.
2. Delete ensure_list_of from model.py.
3. No other changes.

Follow-up decision (out of scope here)

Whether the library should tolerate null collections from the API (mapping them to []) is a real question — the original validators appear to have intended it, but it never worked for the non-optional fields. If desired, that is a BeforeValidator/validate_default-style change and pairs with making ExternalSalesInvoice's collections non-optional (tracked in the detail/payment de-duplication issue).

Acceptance criteria

• The ensure_* validators and ensure_list_of are removed; no replacement added.
• Dicts still become typed models; existing instances preserved; optional fields still preserve None.
• Full test suite passes with no test modifications.