Assets missing from `MjSpec.to_zip`

### Intro

Hi!

### My setup

Version: `mujoco==3.5.0`
API: Python
Architecture: `macos_arm64`

### What's happening? What did you expect?

The `MjSpec.to_zip` [documentation](https://mujoco.readthedocs.io/en/stable/python.html#serialization) says that:
> The `MjSpec` object can be serialized with all of its assets using the function `spec.to_zip(file)`...

I was expecting that I could serialize a model that references external assets (like meshes, textures, etc.) in the spec, e.g. loaded from XML via `` tag. However, `MjSpec.to_zip` only includes assets that are explicitly present in `spec.assets` dictionary. It does not iterate over the model elements (meshes, textures, skins, hfields) to check for file references and include those files. This makes the zip file unusable for most of my use cases.

https://github.com/google-deepmind/mujoco/blob/2efca34f584cd904557774a0825cd1c9c58bb1fb/python/mujoco/__init__.py#L93-L109

### Steps for reproduction

Load any model with "external" assets defined e.g. in the `.xml` `` tag. Dump the model with `spec.to_zip(...)` and try loading it.

### Minimal model for reproduction

_No response_

### Code required for reproduction

_No response_

### Confirmations

- [x] I searched the [latest documentation](https://mujoco.readthedocs.io/en/latest/overview.html) thoroughly before posting.
- [x] I searched previous [Issues](https://github.com/google-deepmind/mujoco/issues) and [Discussions](https://github.com/google-deepmind/mujoco/discussions), I am certain this has not been raised before.