fix(cugraph): filter isolated nodes in CreateCugraphFromMemgraph

Fixes "Index out of range!" error when running cuGraph algorithms on
graphs containing isolated nodes (nodes with no edges).

The bug: CreateCugraphFromMemgraph passed ALL nodes to cuGraph via
cu_vertices, but the GraphView's inner_to_memgraph_id_ mapping only
reliably maps nodes that appear in edges. When cuGraph returned results
for isolated nodes, GetMemgraphNodeId() failed because those node IDs
weren't properly mapped.

The fix: Only include nodes that have at least one edge (in-degree > 0
or out-degree > 0) in the vertex list passed to cuGraph. This is also
semantically correct - isolated nodes have betweenness/PageRank/etc of
zero by definition since no paths traverse them.

Author: TheReaperJay

cpp/cugraph_module/mg_cugraph_utility.hpp

@@ -5,6 +5,7 @@
 
 #pragma once
 
+#include <unordered_set>
 #include <cugraph/algorithms.hpp>
 #include <cugraph/arithmetic_variant_types.hpp>
 #include <cugraph/graph.hpp>
@@ -102,8 +103,19 @@ auto CreateCugraphFromMemgraph(const mg_graph::GraphView<> &mg_graph, const mg_g
   std::transform(
       mg_edges.begin(), mg_edges.end(), std::back_inserter(mg_weight),
       [&mg_graph](const auto &edge) -> TWeightT { return mg_graph.IsWeighted() ? mg_graph.GetWeight(edge.id) : 1.0; });
-  std::transform(mg_nodes.begin(), mg_nodes.end(), std::back_inserter(mg_vertices),
-                 [](const auto &node) -> TVertexT { return node.id; });
+  // Only include nodes that have edges - isolated nodes cause ID mapping issues
+  // and have betweenness = 0 by definition (no paths through them)
+  // Build set of vertices that appear in edges
+  std::unordered_set<TVertexT> vertices_with_edges;
+  for (const auto& edge : mg_edges) {
+    vertices_with_edges.insert(static_cast<TVertexT>(edge.from));
+    vertices_with_edges.insert(static_cast<TVertexT>(edge.to));
+  }
+  for (const auto& node : mg_nodes) {
+    if (vertices_with_edges.count(static_cast<TVertexT>(node.id)) > 0) {
+      mg_vertices.push_back(static_cast<TVertexT>(node.id));
+    }
+  }
 
   // Synchronize the data structures to the GPU
   auto stream = handle.get_stream();