Fix #12: roll back mutations when saving fails

tests/test_manager.py

@@ -556,6 +556,28 @@ class TestHostsManager:
             assert not success
             assert "No sudo permissions" in message
 
+    def test_save_mutation_failure_restores_hosts_file_and_history(self):
+        """The manager owns rollback of model and undo/redo state."""
+        manager = HostsManager()
+        manager.edit_mode = True
+        hosts_file = HostsFile(
+            entries=[HostEntry("192.0.2.1", ["one.test"], is_active=True)]
+        )
+        manager.execute_toggle_command(hosts_file, 0)
+        manager.undo_last_operation(hosts_file)
+        state = manager.capture_mutation_state(hosts_file)
+
+        manager.execute_toggle_command(hosts_file, 0)
+        manager.save_hosts_file = Mock(return_value=(False, "Permission denied"))
+
+        success, message, restored_hosts_file = manager.save_mutation(hosts_file, state)
+
+        assert success is False
+        assert message == "Permission denied"
+        assert restored_hosts_file.entries[0].is_active is True
+        assert not manager.can_undo()
+        assert manager.can_redo()
+
     @patch("subprocess.run")
     def test_restore_backup_success(self, mock_run):
         """Test restoring backup successfully."""