Reject zero nanoeval multiprocessing workers

[sylvesterkaczmarek]

Summary

• reject RunnerArgs(num_processes=0) during configuration validation
• keep None as the default automatic worker-count mode
• preserve the existing requirement that an explicit worker count enables multiprocessing

_validate_multiprocessing_options() currently checks if self.num_processes, so zero skips both validation checks. The value can then reach the multiprocessing executor even though an explicit process count must be positive.

The fix distinguishes None from an explicit integer and validates zero the same way as any other non-positive worker count.

Regression coverage verifies that zero is rejected.