fix(api): match missing-column fallbacks on the DBAPI error, not the statement

str(ProgrammingError) embeds the full SQL, which contains every column
name — the substring check matched any error and could take the wrong
retry branch. Parse the missing column from exc.orig instead.

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

backend/tests/test_sixth_form_flag.py

@@ -148,10 +148,14 @@ def test_load_school_data_survives_missing_has_sixth_form_column(monkeypatch):
     def fake_read_sql(query, con):
         calls.append(query)
         if len(calls) == 1:
+            # The statement text still contains phase_code, school_type_code,
+            # etc. (it's the full _MAIN_QUERY SELECT list) — that's exactly
+            # the collision this test guards against: matching must be done
+            # against exc.orig (the DBAPI error), not str(exc)/the statement.
             raise sqlalchemy.exc.ProgrammingError(
-                "SELECT ...",
-                None,
-                Exception(
+                statement=str(data_loader._MAIN_QUERY),
+                params=None,
+                orig=Exception(
                     "(psycopg2.errors.UndefinedColumn) column s.has_sixth_form "
                     "does not exist"
                 ),