feat(core): define durable datasource artifact contract

tests/test_datasource_contract.py

@@ -3,11 +3,13 @@ from __future__ import annotations
 import unittest
 
 from govoplan_core.core.datasources import (
+    CAPABILITY_DATASOURCE_ARTIFACT_BACKENDS,
     CAPABILITY_DATASOURCE_CATALOGUE,
     CAPABILITY_DATASOURCE_LIFECYCLE,
     CAPABILITY_DATASOURCE_ORIGINS,
     CAPABILITY_DATASOURCE_PUBLICATION,
     DatasourceCatalogueProvider,
+    DatasourceArtifactBackendProvider,
     DatasourceDescriptor,
     DatasourceField,
     DatasourceLifecycleProvider,
@@ -20,6 +22,7 @@ from govoplan_core.core.datasources import (
     DatasourceReadResult,
     DatasourceStage,
     datasource_catalogue,
+    datasource_artifact_backend_provider,
     datasource_lifecycle,
     datasource_origins,
     datasource_publication,
@@ -155,6 +158,9 @@ class _Provider:
             truncated=False,
         )
 
+    def artifact_backends(self):
+        return ()
+
 
 class DatasourceContractTests(unittest.TestCase):
     def test_capabilities_are_runtime_checkable_and_resolved_without_modules(self) -> None:
@@ -163,6 +169,7 @@ class DatasourceContractTests(unittest.TestCase):
         self.assertIsInstance(provider, DatasourceLifecycleProvider)
         self.assertIsInstance(provider, DatasourcePublicationProvider)
         self.assertIsInstance(provider, DatasourceOriginProvider)
+        self.assertIsInstance(provider, DatasourceArtifactBackendProvider)
         registry = PlatformRegistry()
         registry.register(
             ModuleManifest(
@@ -174,6 +181,7 @@ class DatasourceContractTests(unittest.TestCase):
                     CAPABILITY_DATASOURCE_LIFECYCLE: lambda context: provider,
                     CAPABILITY_DATASOURCE_PUBLICATION: lambda context: provider,
                     CAPABILITY_DATASOURCE_ORIGINS: lambda context: provider,
+                    CAPABILITY_DATASOURCE_ARTIFACT_BACKENDS: lambda context: provider,
                 },
             )
         )
@@ -183,6 +191,7 @@ class DatasourceContractTests(unittest.TestCase):
         self.assertIs(provider, datasource_lifecycle(registry))
         self.assertIs(provider, datasource_publication(registry))
         self.assertIs(provider, datasource_origins(registry))
+        self.assertIs(provider, datasource_artifact_backend_provider(registry))
         self.assertIsNone(datasource_catalogue(PlatformRegistry()))
 
     def test_descriptor_distinguishes_mode_kind_shape_and_materialization(self) -> None: