Reads optional simple_metrics_thresholds from model_config and passes
to calculate_simple_metrics. Threshold schema: {rmse_max, r2_min, ...}.
None when not configured (no alerting, no crash).
Updates workflow tests to expect the new key in the activity-call dict.
SIENTIAPDE-1986
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>