Cesium URP Unlit Shader does not render fog

### What happened?

Apparently there is a node in the shader graph to apply fog, but we don't include it in our tileset materials.

(copied from an older issue, needs triage)

### Environment

Cesium for Unity version:
Unity Editor Version:
Operating System:

### Reproduction steps

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### Supporting evidence

_No response_

Start by reproducing the issue in the Unity Editor with the Cesium URP Unlit Shader and a tileset material. Inspect the shader graph's fog node and how fog is applied to tileset materials. Done means fog renders correctly for the affected tileset materials.

Written by the indexing model from the issue text.