Add DATEV accounting adapter boundary

tests/test_accounting_adapters.py

@@ -0,0 +1,103 @@
+from dataclasses import replace
+from decimal import Decimal
+
+import pytest
+
+from fin_hub.accounting import (
+    AccountingCapability,
+    DatevDuoAdapter,
+    SchemaMigrations,
+    default_registry,
+)
+from fin_hub.services.billing import BillingBasisExport, BillingBasisRecord
+
+
+def _export(schema_version: str = "0.1") -> BillingBasisExport:
+    record = BillingBasisRecord(
+        billing_basis_id="billing-basis:stable",
+        cost_attribution_key="client:acme|app:portal|instance:prod-01",
+        client_id="acme",
+        application_id="portal",
+        app_instance_id="prod-01",
+        period_month="2026-07",
+        currency="EUR",
+        price_id="price:agreement-v1",
+        price_revision_of=None,
+        price_source="agreement-v1",
+        revenue=Decimal("200.00"),
+        direct_cost=Decimal("20.00"),
+        allocated_cost=Decimal("80.00"),
+        total_cost=Decimal("100.00"),
+        margin=Decimal("100.00"),
+        financial_fact_ids=("fact:1",),
+        financial_fact_corrections=(),
+        allocation_ids=("allocation:1",),
+        allocation_revisions=(),
+        provenance=("agreement-v1",),
+    )
+    return BillingBasisExport(
+        schema_version=schema_version,
+        artifact_type="billing_basis_report",
+        records=(record,),
+        exceptions=(),
+        disclaimer="Reporting basis only; not an invoice.",
+    )
+
+
+def test_datev_duo_is_default_and_prepares_only_an_incomplete_draft():
+    transfer = default_registry().prepare(_export())
+
+    assert transfer.adapter_id == "datev-duo-v1"
+    assert transfer.provider == "DATEV Unternehmen online"
+    assert transfer.records[0]["external_reference"] == "billing-basis:stable"
+    assert transfer.records[0]["net_billing_basis"] == Decimal("200.00")
+    assert transfer.records[0]["requires_legal_and_tax_completion"] is True
+    assert "Provider draft only" in transfer.disclaimer
+    assert not any(
+        key in transfer.records[0]
+        for key in ("invoice_number", "issue_date", "payment_status")
+    )
+
+
+def test_qonto_requires_explicit_selection_and_keeps_datev_as_destination():
+    transfer = default_registry().prepare(
+        _export(),
+        adapter_id="qonto-invoice-draft-v1",
+        required_capabilities=frozenset(
+            {AccountingCapability.INVOICE_DRAFT_WORKFLOW}
+        ),
+    )
+
+    assert transfer.provider == "Qonto"
+    assert transfer.records[0]["bookkeeping_destination"] == (
+        "DATEV Unternehmen online"
+    )
+
+
+def test_capability_mismatch_never_silently_falls_back_to_qonto():
+    with pytest.raises(ValueError, match="lacks capabilities: invoice_draft_workflow"):
+        default_registry().prepare(
+            _export(),
+            required_capabilities=frozenset(
+                {AccountingCapability.INVOICE_DRAFT_WORKFLOW}
+            ),
+        )
+
+
+def test_breaking_schema_requires_an_explicit_migration():
+    with pytest.raises(ValueError, match="no explicit billing-basis schema migration"):
+        default_registry().prepare(_export("1.0"))
+
+
+def test_registered_schema_migration_must_produce_declared_version():
+    migrations = SchemaMigrations()
+    migrations.register("1.0", "0.1", lambda export: replace(export, schema_version="0.1"))
+    from fin_hub.accounting.adapters import AdapterRegistry
+
+    registry = AdapterRegistry(
+        (DatevDuoAdapter(),),
+        default_adapter_id="datev-duo-v1",
+        migrations=migrations,
+    )
+
+    assert registry.prepare(_export("1.0")).source_schema_version == "0.1"