如何将新字典插入到嵌套字典的最深层？

def insert_dict(nested_dict, new_dict):
    def _insert_dict_helper(current_dict, depth):
        if isinstance(current_dict, dict):
            max_depth = depth
            for key, value in current_dict.items():
                if isinstance(value, dict):
                    nested_depth, nested_dict = _insert_dict_helper(value, depth + 1)
                    if nested_depth > max_depth:
                        max_depth = nested_depth
                else:
                    max_depth = depth
            return max_depth, current_dict
        return depth, None

    max_depth, deepest_dict = _insert_dict_helper(nested_dict, 0)
    if deepest_dict is not None:
        new_key = "new_dict_key"
        deepest_dict[new_key] = new_dict
        return nested_dict
    else:
        raise ValueError("Failed to find the deepest dictionary.")

# 示例使用
nested_dict = {
    "key1": {
        "key2": {
            "key3": {
                "key4": "value4"
            }
        }
    }
}
new_dict = {
    "nested_key1": "nested_value1",
    "nested_key2": "nested_value2"
}

result_dict = insert_dict(nested_dict, new_dict)
print(result_dict)