Tighter frustum culling for 3D Tiles without contentBoundingVolume

https://github.com/CesiumGS/cesium/pull/9172 exposed a case where a draw command is deemed visible by 3D Tiles but is not actually visible when bucketed into frustum commands in `View`. The reason for the discrepancy is that 3D Tiles does its own internal traversal using tile bounding volumes from `tileset.json` rather than using content bounding volumes computed from the glTF content. https://github.com/CesiumGS/cesium/pull/9172#issuecomment-700318187 goes into more details:

> `DrawCommand` has a `cull` property to indicate whether `View` should do frustum culling on the command. All draw commands coming from 3D Tiles will have this set to `false` since 3D Tiles does its own internal frustum culling during traversal and doesn't expect `View` to have to do this check again. The problem is that a tile might be considered visible during traversal but its content might not actually be visible, and this is likely to occur if the bounding volume in `tileset.json` is significantly larger than the bounding volume computed for the content (which is computed from the glTF position accessor min/max). So the command is visible from the tileset's perspective, gets pushed to the command list, goes through `View`, modifies camera near/far and shadow near/far, but ultimately doesn't get placed into any frustum command lists because it's behind all the frustums. As a result it doesn't get rendered but it does move the near plane of the camera and shadows much nearer than necessary.

A possible fix would be for 3D Tiles to use the tile's bounding volume for traversal and the draw command's bounding volume for selection (as long as the tile content is loaded). Normally 3D Tiles uses `contentBoundingVolume` for tighter culling but it's not always present in `tileset.json`.

Start by tracing 3D Tiles traversal into View's frustum-command bucketing, focusing on DrawCommand.cull and the tile and content bounding volumes described in the issue. Verify how loaded content without a contentBoundingVolume affects selection, near/far updates, and final rendering, then confirm that invisible commands no longer influence those values.

Written by the indexing model from the issue text.