HuggingFace model browser: robustness cleanups in listTasks fan-out and hasModelsForTask

### Task Summary

Two robustness issues in the model-browser REST resource that backs the operator's task/model dropdowns.

### Context / checklist
- [ ] `listTasks` fans out one HF request per pipeline tag on every cache miss, and skips caching entirely when a user token is present, heavy and easily rate-limited.
- Rate-limited (`429`/`503`) `hasModelsForTask` checks silently drop tasks from the returned list, so transient HF throttling makes valid tasks disappear from the dropdown.

File: `amber/.../web/resource/HuggingFaceModelResource.scala` (`listTasks`, `hasModelsForTask`).

### Required test
Per item, as addressed (extend `HuggingFaceModelResourceSpec`).

### Task Type

- [x] Refactor / Cleanup
- [ ] DevOps / Deployment / CI
- [ ] Testing / QA
- [ ] Documentation
- [ ] Performance
- [ ] Other