Maayan via Elementary: Add data contract tests for cpa_and_roas

[joostboon]

Data Contract Enforcement for cpa_and_roas

This PR adds comprehensive data quality tests to enforce a data contract on the cpa_and_roas model:

Tests Added:

1. Freshness: dbt_utils.recency on day — ensures data includes records from at least yesterday
2. Completeness: not_null on 5 core columns (day, utm_source, total_spend, attribution_points, attribution_revenue)
3. Uniqueness: dbt_utils.unique_combination_of_columns on (day, utm_source) — enforces the table grain
4. Anomaly Detection: elementary.column_anomalies on cost_per_acquisition (average) — monitors CPA for unexpected shifts

Already Covered (not modified):

• ✅ Freshness anomaly (cloud monitor)
• ✅ Volume anomaly (cloud monitor)
• ✅ ROAS anomaly (column_anomalies on return_on_advertising_spend)
  
  Created by: maayan+172@elementary-data.com