fix(metrics): correct never-tested technique query [FASE-2.6]

Use distinct technique_id list filtering so untested techniques are returned reliably on SQLite and Postgres.

backend/app/services/advanced_metrics_service.py

@@ -51,13 +51,17 @@ def get_coverage_by_tactic(db: Session) -> list[dict]:
 
 def get_never_tested_techniques(db: Session) -> list[dict]:
     """Techniques that have never had a test created."""
-    tested_technique_ids = db.query(Test.technique_id).distinct().subquery()
-    techniques = (
-        db.query(Technique)
-        .filter(~Technique.id.in_(db.query(tested_technique_ids)))
-        .order_by(Technique.mitre_id)
+    tested_ids = [
+        row[0]
+        for row in db.query(Test.technique_id)
+        .filter(Test.technique_id.isnot(None))
+        .distinct()
         .all()
-    )
+    ]
+    query = db.query(Technique)
+    if tested_ids:
+        query = query.filter(~Technique.id.in_(tested_ids))
+    techniques = query.order_by(Technique.mitre_id).all()
     return [
         {
             "mitre_id": t.mitre_id,