Allow X wildcard

Author: PerthCharern

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

@@ -41,7 +41,7 @@ public static class OpenApiResponsesRules
                     {
                         context.Enter(key);
 
-                        if (key != "default" && !Regex.IsMatch(key, "^[1-5][0-9][0-9]$"))
+                        if (key != "default" && !Regex.IsMatch(key, "^[1-5|X][0-9|X][0-9|X]$"))
                         {
                             context.AddError(
                                 new ValidationError(

test/Microsoft.OpenApi.Tests/Validations/ValidationRuleSetTests.cs

@@ -43,7 +43,7 @@ public void DefaultRuleSetPropertyReturnsTheCorrectRules()
             Assert.NotEmpty(rules);
 
             // Update the number if you add new default rule(s).
-            Assert.Equal(14, rules.Count);
+            Assert.Equal(15, rules.Count);
         }
     }
 }