MVTDataProvider: Tileset loading stalls if any tile request returns an error

### What happened?

Reported by an MVTDataProvider user — When serving .mvt files dynamically from a server, tile requests may occasionally fail or time out. When this happens, e.g. any tile returns an HTTP 500 error, the client discontinues loading, and working tiles are not loaded either.

### Reproduction steps

Reproduction is needed, but not yet available. We may be able to reproduce this with a small monkey-patch in the request code to throw an error on the 20th request or something like that.

### Sandcastle example

Not available.

### Environment

Browser: not available
CesiumJS Version: not available
Operating System: not available

### 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 at the MVTDataProvider tile-loading and request-handling entry points, then trace how a failed tile request affects the remaining requests. Reproduce the failure, ideally by forcing an individual request to return an error, and add regression coverage showing that successful tiles continue loading after one failure.

Written by the indexing model from the issue text.