Add test for Graph vertex deletion with loop

src/models/graph.rs

@@ -479,6 +479,36 @@ mod tests {
         );
     }
 
+    #[test]
+    fn delete_vertex_loop() {
+        let mut graph = Graph::new();
+        let v = graph.add_vertex();
+        graph.add_edge(v, v);
+        assert_eq!(
+            graph.vertex_count(),
+            1,
+            "unexpected vertex count before delete"
+        );
+        assert_eq!(graph.edge_count(), 1, "unexpected edge count before delete");
+        assert!(
+            graph.are_adjacent(v, v),
+            "expected vertex to be self-adjacent before delete"
+        );
+        assert_eq!(graph.degree(v), 2, "unexpected vertex degree before delete");
+        graph.delete_vertex(v);
+        assert_eq!(
+            graph.vertex_count(),
+            0,
+            "unexpected vertex count after delete"
+        );
+        assert_eq!(graph.edge_count(), 0, "unexpected edge count after delete");
+        assert_ne!(
+            graph.add_vertex(),
+            v,
+            "unexpected duplicate vertex after delete"
+        );
+    }
+
     #[test]
     fn delete_vertex_invalid_index() {
         let mut graph = Graph::new();