Separate warnings and errors in Open API YAML reader ReadAsync method

Author: millicentachieng

src/Microsoft.OpenApi.Readers/OpenApiYamlDocumentReader.cs

@@ -114,11 +114,15 @@ public async Task<ReadResult> ReadAsync(YamlDocument input)
             // Validate the document
             if (_settings.RuleSet != null && _settings.RuleSet.Rules.Count > 0)
             {
-                var errors = document.Validate(_settings.RuleSet);
-                foreach (var item in errors)
+                var openApiErrors = document.Validate(_settings.RuleSet);
+                foreach (var item in openApiErrors.Where(e => e is OpenApiValidatorError))
                 {
                     diagnostic.Errors.Add(item);
                 }
+                foreach (var item in openApiErrors.Where(e => e is OpenApiValidatorWarning))
+                {
+                    diagnostic.Warnings.Add(item);
+                }
             }
 
             return new ReadResult()