fix: enforce durable migration ownership and readiness

tests/test_migration_environment.py

@@ -0,0 +1,12 @@
+from __future__ import annotations
+
+import pytest
+
+from hub_core.migrations.roles import migration_role_statement
+
+
+def test_migration_role_is_quoted_and_validated() -> None:
+    assert migration_role_statement("hub_runtime_owner") == 'SET ROLE "hub_runtime_owner"'
+    assert migration_role_statement(None) is None
+    with pytest.raises(ValueError, match="safe PostgreSQL role"):
+        migration_role_statement('owner"; DROP SCHEMA public; --')