Harden BYOB config validation

[fallintoplace]

Summary

• replace BYOB runtime config assert statements with explicit validation that raises ValueError
• apply the same treatment to the translation config loader so python -O does not silently disable those checks
• add focused regression coverage for missing fields, threshold validation, and the translation FAITH-eval guard

Root cause

ByobConfig.from_yaml() and ByobTranslationConfig.from_yaml() used assert for user-facing config validation. Under python -O, those assertions are stripped entirely, which means required-field and threshold checks can disappear at runtime.

What changed

I added a small _require() helper in src/nemotron/steps/byob/runtime/config.py and converted the runtime config validators over to explicit checks. The error messages stay the same, but config failures now consistently surface as ValueError even when Python optimization is enabled.

Validation

• python3 -m py_compile src/nemotron/steps/byob/runtime/config.py tests/steps/byob/test_byob_config.py
• uv run --no-project --with pytest --with numpy --with pyyaml --with datasets --with requests env PYTHONPATH=src pytest tests/steps/byob/test_byob_config.py

[chrisalexiuk-nvidia]

Need signoff.

[fallintoplace]

@chrisalexiuk-nvidia Let me have a look and push this.