fix(pipeline): normalize GIAS OfficialSixthForm comparison with lower(trim())

Matches the phase derivation's guard against casing/whitespace variants in
raw GIAS data; an unmatched variant previously fell through silently to the
statutory-age fallback.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pipeline/transform/models/marts/dim_school.sql

@@ -55,9 +55,12 @@ select
     -- Authoritative sixth-form flag (spec §3): GIAS OfficialSixthForm.
     -- "Not applicable" (nurseries, primaries, PRUs) => false. Blank GIAS
     -- value (rare, new establishments) falls back to the statutory age range.
+    -- lower(trim()) guards against casing/whitespace variants in raw GIAS
+    -- data, same as the phase derivation above — an unmatched variant would
+    -- otherwise silently fall through to the age-range fallback.
     case
-        when s.official_sixth_form = 'Has a sixth form' then true
-        when s.official_sixth_form in ('Does not have a sixth form', 'Not applicable') then false
+        when lower(trim(s.official_sixth_form)) = 'has a sixth form' then true
+        when lower(trim(s.official_sixth_form)) in ('does not have a sixth form', 'not applicable') then false
         else coalesce(s.statutory_high_age >= 18, false)
     end as has_sixth_form,
     s.capacity,