test: drop isinstance(SessionStore) check in Postgres test (missed in fdb202e)

Author: qing-ant

tests/test_example_postgres_session_store.py

@@ -136,8 +136,14 @@ async def make_store() -> SessionStore:
             for t in tables:
                 await pool.execute(f"DROP TABLE IF EXISTS {t}")
 
-    def test_isinstance_session_store(self, store: SessionStore) -> None:
-        assert isinstance(store, SessionStore)
+    def test_store_implements_required_methods(self, store: SessionStore) -> None:
+        """SessionStore is not @runtime_checkable; probe via _store_implements()."""
+        from claude_agent_sdk._internal.session_store_validation import (
+            _store_implements,
+        )
+
+        assert _store_implements(store, "append")
+        assert _store_implements(store, "load")
 
     def test_rejects_unsafe_table_name(self, pool: asyncpg.Pool) -> None:
         with pytest.raises(ValueError, match="must match"):