Gaussian Splatting Dynamic Loading Issue – BoundingBox disappears at distance but model remains visible

### What happened?

When loading a Gaussian splatting 3D Tileset in Cesium with debugShowBoundingVolume = true, an inconsistent behavior occurs:

At close range: Both the bounding boxes and the Gaussian model are rendered correctly.

At far distance: The bounding boxes disappear (as expected, likely due to LOD/culling), but the Gaussian model itself remains visible instead of being culled or switching to a lower LOD representation.

### Reproduction steps

1、Load a 3D Tileset containing Gaussian splats (e.g., converted with KHR_gaussian_splatting extension).

2、Enable debug bounding volumes: tileset.debugShowBoundingVolume = true;

3、Zoom camera to close distance – both bounding boxes and model visible.

4、Zoom camera far away – bounding boxes disappear, but model stays visible.

### Sandcastle example

_No response_

### Environment

Browser: google
CesiumJS Version: 1.140
Operating System: windows

### AI acknowledgment

- [ ] I used AI to generate this issue report.
- [ ] (If the above is checked) I have reviewed the AI-generated content before submitting.

Start by reproducing the behavior with a Gaussian splatting 3D Tileset in CesiumJS 1.140, enabling tileset.debugShowBoundingVolume and comparing close- and far-distance visibility. Done means determining why the Gaussian model remains visible after its bounding boxes disappear and making its culling or LOD behavior consistent.

Written by the indexing model from the issue text.