Add test to verify

Author: MaggieKimani1

test/Microsoft.OpenApi.Tests/Models/OpenApiSchemaTests.cs

@@ -1,15 +1,18 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license.
 
+using System;
 using System.Collections.Generic;
 using System.Globalization;
 using System.IO;
+using System.Linq;
 using System.Threading.Tasks;
 using FluentAssertions;
 using Microsoft.OpenApi.Any;
 using Microsoft.OpenApi.Extensions;
 using Microsoft.OpenApi.Interfaces;
 using Microsoft.OpenApi.Models;
+using Microsoft.OpenApi.Services;
 using Microsoft.OpenApi.Writers;
 using VerifyXunit;
 using Xunit;
@@ -506,5 +509,56 @@ public void CloningSchemaExtensionsWorks()
             };
             Assert.NotEqual(schema.Extensions, schemaCopy.Extensions);
         }
+
+        [Fact]
+        public void OpenApiWalkerVisitsOpenApiSchemaNot()
+        {
+            var outerSchema = new OpenApiSchema()
+            {
+                Title = "Outer Schema",
+                Not = new OpenApiSchema()
+                {
+                    Title = "Inner Schema",
+                    Type = "string",
+                }
+            };        
+
+            var document = new OpenApiDocument()
+            {
+                Paths = new OpenApiPaths()
+                {
+                    ["/foo"] = new OpenApiPathItem()
+                    {
+                        Parameters = new[]
+                        {
+                            new OpenApiParameter()
+                            {
+                                Name = "foo",
+                                In = ParameterLocation.Query,
+                                Schema = outerSchema,
+                            }
+                        }
+                    }
+                }
+            };
+
+            // Act
+            var visitor = new SchemaVisitor();
+            var walker = new OpenApiWalker(visitor);
+            walker.Walk(document);
+
+            // Assert
+            visitor.Titles.Count.Should().Be(2);
+        }
+    }
+
+    internal class SchemaVisitor : OpenApiVisitorBase
+    {
+        public List<string> Titles = new();
+
+        public override void Visit(OpenApiSchema schema)
+        {
+            Titles.Add(schema.Title);
+        }
     }
 }