fix: reject zero multi-GPU workers

[nkbeast]

Summary

• reject n_devices < 1 before splitting empty or non-empty workloads
• add a regression test for zero-worker input

Validation

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

[Intron7]

There will be another error on this not needed