Fix availability check in VoxelTraversal for procedural tilesets

[jjhembd]

Description

This PR fixes a small bug introduced in #12430 affecting procedural tilesets.

The requestData helper function in VoxelTraversal included a check of provider._implicitTileset.availableLevels to avoid generating spurious tileFailed events. However, custom procedural providers (as used in some of our sandcastle examples) do not have an ._implicitTileset.

This PR adds a public availableLevels property to VoxelProvider, which is allowed to be undefined. VoxelTraversal now verifies the property is defined before checking its value.

Issue number and link

Testing plan

Load the "Voxels" Sandcastle and verify that all datasets render without errors.

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• [ ] I have updated CHANGES.md with a short summary of my change Fixes a bug not present in the previous release
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @jjhembd!

✅ We can confirm we have a CLA on file for you.

[ggetz]

Thanks for the fix @jjhembd!

Would it be possible to add a unit test for this case to prevent a similar regression in the future? I'm surprised we don't have one that failed already!

[jjhembd]

Would it be possible to add a unit test for this case to prevent a similar regression in the future? I'm surprised we don't have one that failed already!

@ggetz The specs that actually loaded tiles were all rendering one of the test 3D Tiles voxel datasets. I added a spec to load a minimal procedural provider. This spec fails in main, and succeeds in this branch.

[lukemckinstry]

@jjhembd this fix looks good to me

[ggetz]

Thanks @jjhembd and @lukemckinstry!