fix: ensure cancellation token is properly observed after Parallel.ForEachAsync

Author: MPCoreDeveloper

src/SharpCoreDB.Graph/ParallelGraphTraversalEngine.cs

@@ -129,6 +129,9 @@ await Parallel.ForEachAsync(
                     }
                 });
 
+            // ✅ Check cancellation after parallel work completes
+            cancellationToken.ThrowIfCancellationRequested();
+
             currentLevel = nextLevel;
         }