fix: throw on circular reference

Author: baywet

src/Microsoft.OpenApi/Services/OpenApiWorkspace.cs

@@ -376,7 +376,10 @@ public bool Contains(string location)
             // Prevent infinite recursion in case of circular references
             if (visitedSchemas.Contains(schema))
             {
-                return null;
+                if (schema is OpenApiSchemaReference openApiSchemaReference)
+                    throw new InvalidOperationException($"Circular reference detected while resolving schema: {openApiSchemaReference.Reference.ReferenceV3}");
+                else
+                    throw new InvalidOperationException($"Circular reference detected while resolving schema");
             }
             visitedSchemas.Push(schema);
             // Traverse schema object to resolve subschemas

test/Microsoft.OpenApi.Readers.Tests/V31Tests/RelativeReferenceTests.cs

@@ -502,9 +502,9 @@ public void ExitsEarlyOnCyclicalReferences()
             document.RegisterComponents();
 
             var tagsSchemaRef = Assert.IsType<OpenApiSchemaReference>(categorySchema.Properties["tags"]);
-            Assert.Null(tagsSchemaRef.Items);
+            Assert.Throws<InvalidOperationException>(() => tagsSchemaRef.Items);
             Assert.Equal("#/components/schemas/Category/properties/parent/properties/tags", tagsSchemaRef.Reference.ReferenceV3);
-            Assert.Null(tagsSchemaRef.Target);
+            Assert.Throws<InvalidOperationException>(() => tagsSchemaRef.Target);
 
             var parentSchemaRef = Assert.IsType<OpenApiSchemaReference>(categorySchema.Properties["parent"]);
             Assert.Equal("#/components/schemas/Category", parentSchemaRef.Reference.ReferenceV3);