Add validation for server variable default value

Author: calebkiage

src/Microsoft.OpenApi/Validations/Rules/OpenApiServerRules.cs

@@ -26,9 +26,31 @@ public static class OpenApiServerRules
                         context.CreateError(nameof(ServerRequiredFields),
                             String.Format(SRResource.Validation_FieldIsRequired, "url", "server"));
                     }
+
                     context.Exit();
+                    context.Enter("variables");
+                    foreach (var variable in server.Variables)
+                    {
+                        context.Enter(variable.Key);
+                        ValidateServerVariableRequiredFields(context, variable.Value);
+                        context.Exit();
+                    }
                 });
 
         // add more rules
+
+        /// <summary>
+        /// Validate required fields in server variable
+        /// </summary>
+        private static void ValidateServerVariableRequiredFields(IValidationContext context, OpenApiServerVariable item)
+        {
+            context.Enter("default");
+            if (string.IsNullOrEmpty(item.Default))
+            {
+                context.CreateError("ServerVariableMustHaveDefaultValue",
+                    String.Format(SRResource.Validation_FieldIsRequired, "default", "variables"));
+            }
+            context.Exit();
+        }
     }
 }

test/Microsoft.OpenApi.Readers.Tests/V3Tests/OpenApiDocumentTests.cs

@@ -1413,7 +1413,7 @@ public void ParseBasicDocumentWithServerVariableShouldSucceed()
                 });
         }
 
-        [Fact(Skip = "This test requires a validation for the Server variable object to be added.")]
+        [Fact]
         public void ParseBasicDocumentWithServerVariableAndNoDefaultShouldFail()
         {
             var openApiDoc = new OpenApiStringReader().Read("""