Fix adapter-only validation adapter runs

[bghira]

Summary

• honor validation_adapter_mode for validation_adapter_config runs
• only add a base validation run for comparison mode or when no adapter runs are configured
• ignore adapter config entries when adapter mode is none

Tests

• .venv/bin/python -m unittest tests.test_validation_adapters -v
• .venv/bin/python -m unittest -v -f

Fixes #3184