Standardize identifier field typing across models

[TimEvci]

Summary

Identifier fields are typed inconsistently across models: id and version are int in most models but str | int in FinancialMutation. The official OpenAPI spec types identifiers as string-or-integer matching ^\d+$, and real responses send them as strings of 64-bit numbers (e.g. "433546185192506620", as seen throughout tests/conftest.py). The int-typed fields currently rely on pydantic silently coercing string → int.

Decision (made): treat identifiers as str

All identifier fields (id, version, *_id) should converge on str:

• It matches what the API actually sends.
• Identifiers are opaque tokens, not quantities — nothing meaningful is gained by int.
• It removes the silent string→int coercion and the str | int unions.

Inputs should still accept ints for ergonomics (SalesInvoice.find_by_id(123)), coercing to str internally — pydantic can do this with a lenient annotated type, e.g.:

Identifier = Annotated[str, BeforeValidator(lambda v: str(v) if v is not None else None)]

Classmethod signatures that take ids (find_by_id, update_by_id, delete_by_id, sync_fetch, get_detail, delete_payment, ...) should widen to int | str.

⚠️ Breaking change

This changes the public type of model.id and friends: code doing invoice.id == 12345 or passing ids into int-typed downstream code will break. This should land in a major/minor version bump with a clear changelog entry, not a patch release.

Scope

• moneysnake/model.py (MoneybirdModel.id, id-taking classmethods)
• Every model with *_id / version fields (sales_invoice.py, document.py, external_sales_invoice.py, contact.py, financial_mutation.py, financial_statement.py, payment.py, tax_rate.py, detail-attribute classes)
• Tests asserting integer ids

Acceptance criteria

• All identifier fields are str | None (via the shared annotated type); no remaining str | int unions for ids.
• Constructing a model from a spec-shaped response (string ids) and from int ids both yield string identifiers.
• URL building (f"...{self.id}") and request bodies are unaffected in behavior.
• Tests updated; changelog notes the breaking change.