Exported XML loses meshdir and relative mesh paths

I'm using PyMJCF to load a robot model from an XML file and build a MuJoCo scene. When I export the composed scene back to an XML string using `scene_model.to_xml_string()`, I've run into an issue with the file paths.

### The Problem

The exported XML file is missing the directive. Meanwhile, all the mesh file paths, which were originally relative to that directory, are now missing their path prefixes.

For example, a mesh defined like `file="my_robot/meshes/part1.stl"` becomes `file="part1.stl"` in the exported file.

### Code Snippet

Here is the line of code I'm using for the export:

```Python
final_xml_string = scene_model.to_xml_string()
```

### What I've Found

I found a related issue (#454) that suggests using mjcf.export_with_assets(...) as a potential solution for managing assets.

### My Questions

1. Is there a direct method in PyMJCF to preserve the meshdir compiler attribute and the corresponding relative file paths when exporting a model to an XML string?

2. Separately, my main goal is to visualize the composed scene. I know `dm_control.viewer` can be used for simulations. Does the viewer also support loading and visualizing a custom MJCF scene like the one I'm building?

Thanks for your help!

从 issue 中描述的 `scene_model.to_xml_string()` 调用开始，并将其行为与 issue #454 中相关的 `mjcf.export_with_assets(...)` 路径进行比较。检查导出是否能够保留 `meshdir` 和相对 mesh 路径，并单独调查 `dm_control.viewer` 是否接受组合后的场景。只有对这两个问题都有文档记录或经过测试的答案，才算完成。

由索引模型根据 Issue 内容生成。