Fix some failing tests

Update error message for server variable

Author: calebkiage

src/Microsoft.OpenApi/Extensions/OpenApiServerExtensions.cs

@@ -30,6 +30,7 @@ public static string ReplaceServerUrlVariables(this OpenApiServer server, IDicti
             values.TryGetValue(variable.Key, out var actualValue);
             // Fall back to the default value
             if (string.IsNullOrEmpty(actualValue)) { actualValue = variable.Value.Default; }
+
             if (string.IsNullOrEmpty(actualValue))
             {
                 // According to the spec, the variable's default value is required.
@@ -38,13 +39,15 @@ public static string ReplaceServerUrlVariables(this OpenApiServer server, IDicti
                     string.Format(SRResource.ParseServerUrlDefaultValueNotAvailable, variable.Key), nameof(server));
             }
 
-            if (variable.Value.Enum?.Contains(actualValue) != true)
+            if (variable.Value.Enum is { Count: > 0 } e && !e.Contains(actualValue))
             {
                 throw new ArgumentException(
                     string.Format(SRResource.ParseServerUrlValueNotValid, actualValue, variable.Key), nameof(values));
             }
+
             parsedUrl = parsedUrl.Replace($"{{{variable.Key}}}", actualValue);
         }
+
         return parsedUrl;
     }
 }

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

@@ -32,23 +32,24 @@ public static class OpenApiServerRules
                     foreach (var variable in server.Variables)
                     {
                         context.Enter(variable.Key);
-                        ValidateServerVariableRequiredFields(context, variable.Value);
+                        ValidateServerVariableRequiredFields(context, variable.Key, variable.Value);
                         context.Exit();
                     }
+                    context.Exit();
                 });
 
         // add more rules
 
         /// <summary>
         /// Validate required fields in server variable
         /// </summary>
-        private static void ValidateServerVariableRequiredFields(IValidationContext context, OpenApiServerVariable item)
+        private static void ValidateServerVariableRequiredFields(IValidationContext context, string key, OpenApiServerVariable item)
         {
             context.Enter("default");
             if (string.IsNullOrEmpty(item.Default))
             {
                 context.CreateError("ServerVariableMustHaveDefaultValue",
-                    String.Format(SRResource.Validation_FieldIsRequired, "default", "variables"));
+                    String.Format(SRResource.Validation_FieldIsRequired, "default", key));
             }
             context.Exit();
         }