chore: exposes unused property so consumers can read the information

Signed-off-by: Vincent Biret <[email protected]>

Author: baywet

src/Microsoft.OpenApi/Services/OpenApiReferenceError.cs

@@ -11,7 +11,10 @@ namespace Microsoft.OpenApi.Services
     /// </summary>
     public class OpenApiReferenceError : OpenApiError
     {
-        private OpenApiReference _reference;
+        /// <summary>
+        /// The reference that caused the error.
+        /// </summary>
+        public readonly OpenApiReference Reference;
         /// <summary>
         /// Initializes the <see cref="OpenApiError"/> class using the message and pointer from the given exception.
         /// </summary>
@@ -26,7 +29,7 @@ public OpenApiReferenceError(OpenApiException exception) : base(exception.Pointe
         /// <param name="message"></param>
         public OpenApiReferenceError(OpenApiReference reference, string message) : base("", message)
         {
-            _reference = reference;
+            Reference = reference;
         }
     }
 }