fix(pipeline): expand GIAS schema, handle empty strings, scope DAG selectors

- Declare all 34 columns needed by dbt in GIAS tap schema (target-postgres
  only persists columns present in the Singer schema message)
- Use nullif() for empty-string-to-integer/date casts in staging models
- Scope daily DAG dbt build to GIAS models only (stg_gias_establishments+
  stg_gias_links+) to avoid errors on unloaded sources
- Scope annual EES DAG similarly; remove redundant dbt test steps
- Make dim_school gracefully handle missing int_ofsted_latest table

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

pipeline/transform/models/marts/dim_school.sql

@@ -2,12 +2,14 @@
 
 with schools as (
     select * from {{ ref('stg_gias_establishments') }}
-),
-
-latest_ofsted as (
-    select * from {{ ref('int_ofsted_latest') }}
 )
 
+{% set ofsted_relation = adapter.get_relation(
+    database=target.database,
+    schema=target.schema,
+    identifier='int_ofsted_latest'
+) %}
+
 select
     s.urn,
     s.local_authority_code * 1000 + s.establishment_number as laestab,
@@ -30,11 +32,19 @@ select
     s.nursery_provision,
     s.admissions_policy,
 
-    -- Latest Ofsted
+    -- Latest Ofsted (populated after monthly Ofsted pipeline runs)
+    {% if ofsted_relation is not none %}
     o.overall_effectiveness as ofsted_grade,
     o.inspection_date as ofsted_date,
     o.framework as ofsted_framework
+    {% else %}
+    null::text as ofsted_grade,
+    null::date as ofsted_date,
+    null::text as ofsted_framework
+    {% endif %}
 
 from schools s
-left join latest_ofsted o on s.urn = o.urn
+{% if ofsted_relation is not none %}
+left join {{ ref('int_ofsted_latest') }} o on s.urn = o.urn
+{% endif %}
 where s.status = 'Open'