Incorrect depth sorting between Gaussian models at different heights in the same area

### What happened?

When loading two identical Gaussian models at different heights within the same geographic area, the lower model incorrectly occludes the upper model at certain camera angles.

Specifically, when rotating the camera to particular viewing directions, parts of the model placed at a lower altitude are rendered in front of the model placed at a higher altitude, even though they should be fully occluded based on their vertical separation.

https://github.com/user-attachments/assets/e64e5f1f-83b7-49f9-a39c-58041ede26a2

### Reproduction steps

1. Load two identical Gaussian models in the same longitude/latitude location.
2. Assign different heights (e.g., one at ground level and one several meters above).
3. Rotate the camera around the models.
4. Observe that at specific viewing angles, the lower model incorrectly occludes the upper one.

### Sandcastle example

https://sandcastle.cesium.com/#c=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

### Environment

Browser: Chrome 145.0.7632.110
CesiumJS Version: 1.139.0
Operating System: windows 11

Start by running the linked Sandcastle example with two identical Gaussian models at different heights, then rotate the camera through the angles that reproduce the occlusion. Trace the Gaussian model rendering and depth-sorting behavior; the issue is done when the lower model no longer incorrectly renders in front of the higher model at any viewing angle.

Written by the indexing model from the issue text.