test(dim_school): downgrade phase not_null to warn

The new phase inference can legitimately leave ~1100 independent schools with
null phase (no GIAS phase, no statutory ages, name gives no hint). That's a
known data quality gap, not a pipeline failure — the UI already handles null
by showing no pill. Downgrade the test to warn so it stays visible in dbt
output without blocking the DAG.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pipeline/transform/models/marts/_marts_schema.yml

@@ -9,7 +9,13 @@ models:
       - name: school_name
         tests: [not_null]
       - name: phase
-        tests: [not_null]
+        description: >
+          Primary / Secondary / All-through etc. May be null for a small number
+          of independent schools where GIAS publishes "Not Applicable", no
+          statutory age range, and the school name gives no hint.
+        tests:
+          - not_null:
+              severity: warn
       - name: status
         tests:
           - accepted_values: