Merged PR 5261: Adds contract signed month in onboarding reporting

# Description

Adds contract signed month + not null test

# Checklist

- [X] The edited models and dependants run properly with production data.
- [X] The edited models are sufficiently documented.
- [X] The edited models contain PK tests, and I've ran and passed them.
- [ ] I have checked for DRY opportunities with other models and docs.
- [ ] I've picked the right materialization for the affected models.

# Other

- [ ] Check if a full-refresh is required after this PR is merged.

Related work items: #30249

models/reporting/general/schema.yml

@@ -2868,6 +2868,16 @@ models:
         data_type: date
         description: |
           Date in which the contract was signed according to HubSpot.
+        data_tests:
+          - not_null
+
+      - name: contract_signed_month
+        data_type: date
+        description: |
+          First day of the month in which the contract was signed according 
+          to HubSpot.
+        data_tests:
+          - not_null
 
       - name: live_date_utc
         data_type: date