fix: validate multi-GPU device selections

[nkbeast]

Summary

• reject multi-GPU selections when no CUDA devices are available
• validate list-based device IDs before comparing them with device bounds
• add focused regression tests for both invalid states

Validation

• ruff check — passed
• python -m compileall — passed
• git diff --check — passed
• focused pytest collection is blocked because this environment lacks scanpy

[Intron7]

RSC always needs GPUs cupy will fail earlier this defence is not needed