ci: stop hardcoding 'snapshots' target_schema in failed_snapshot fixture (CORE-687)

failed_snapshot.sql is a parse-time fixture (intentionally invalid SQL)
that's never actually run by CI. The hardcoded target_schema='snapshots'
was forcing a fixed Glue database name and breaking the per-run schema
isolation pattern.  Use target.schema instead so it lands in the same
ephemeral schema as the rest of the run.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: haritamar

tests/e2e_dbt_project/snapshots/failed_snapshot.sql

@@ -1,8 +1,9 @@
 {% snapshot failed_snapshot() %}
 
+{# target_schema is required by dbt; reuse target.schema so this lands in the per-run CI schema. #}
 {{
     config(
-      target_schema='snapshots',
+      target_schema=target.schema,
       unique_key='unique_id',
       strategy='timestamp',
       updated_at='generated_at',