feat(core): add datasource lifecycle governance contracts

tests/test_datasource_contract.py

@@ -13,6 +13,7 @@ from govoplan_core.core.datasources import (
     DatasourceArtifactBackendProvider,
     DatasourceDescriptor,
     DatasourceField,
+    DatasourceGovernance,
     DatasourceLifecycleProvider,
     DatasourceMaterialization,
     DatasourceOrigin,
@@ -212,6 +213,55 @@ class DatasourceContractTests(unittest.TestCase):
         self.assertEqual("upload", descriptor.kind)
         self.assertEqual("tabular", descriptor.shape)
 
+    def test_lifecycle_governance_round_trips_without_provider_specific_types(self) -> None:
+        governance = DatasourceGovernance.from_mapping(
+            {
+                "approval_policy": {
+                    "version": "approval-v2",
+                    "required": True,
+                    "required_approvals": 2,
+                },
+                "retention_policy": {
+                    "version": "retention-v3",
+                    "enabled": True,
+                    "stage_days": 30,
+                },
+            }
+        )
+
+        self.assertEqual("approval-v2", governance.approval_policy["version"])
+        self.assertEqual(30, governance.retention_policy["stage_days"])
+        self.assertEqual(
+            governance.approval_policy,
+            governance.to_dict()["approval_policy"],
+        )
+        self.assertEqual(
+            governance.retention_policy,
+            governance.to_dict()["retention_policy"],
+        )
+
+        stage = DatasourceStage(
+            ref="stage:governed",
+            name="Governed stage",
+            source_name="governed",
+            kind="upload",
+            mode="static",
+            shape="tabular",
+            state="awaiting_approval",
+            approval={"status": "pending", "policy_version": "approval-v2"},
+        )
+        materialization = DatasourceMaterialization(
+            ref="materialization:disposed",
+            datasource_ref="datasource:governed",
+            revision=1,
+            state="disposed",
+            fingerprint="abc123",
+            disposition={"reason": "retention_policy", "policy_version": "retention-v3"},
+        )
+
+        self.assertEqual("pending", stage.approval["status"])
+        self.assertEqual("retention_policy", materialization.disposition["reason"])
+
 
 if __name__ == "__main__":
     unittest.main()