fix: add check for key (#1408)

Author: z3z1ma

sqlmesh/migrations/v0023_fix_added_models_with_forward_only_parents.py

@@ -36,6 +36,8 @@ def migrate(state_sync: t.Any) -> None:
     snapshots_to_delete = set()
 
     for snapshot_id in dag:
+        if snapshot_id not in snapshot_mapping:
+            continue
         parsed_snapshot = snapshot_mapping[snapshot_id]
         is_breaking = parsed_snapshot.get("change_category") == 1
         has_previous_versions = bool(parsed_snapshot.get("previous_versions", []))