Centralize error messages in Constants/ErrorMessages.cs

- Created new ErrorMessages.cs constants file for centralized error message management
- Updated RTEJsonConverter.cs to use error message constant instead of hardcoded string
- Added Constants folder to project file
- Improves maintainability by keeping all error messages in one location

Author: reeshika-h

Contentstack.Utils/Constants/ErrorMessages.cs

@@ -0,0 +1,11 @@
+namespace Contentstack.Utils.Constants
+{
+    /// <summary>
+    /// Centralized error messages for the Contentstack Utils library
+    /// </summary>
+    public static class ErrorMessages
+    {
+        public const string InvalidRteJson = "Invalid RTE JSON. Provide a valid JSON structure and try again.";
+    }
+}
+

Contentstack.Utils/Contentstack.Utils.csproj

@@ -36,6 +36,7 @@
     <Folder Include="Enums\" />
     <Folder Include="Extensions\" />
     <Folder Include="Converters\" />
+    <Folder Include="Constants\" />
   </ItemGroup>
   <ItemGroup>
     <PackageReference Include="HtmlAgilityPack" Version="1.11.46" />

Contentstack.Utils/Converters/RTEJsonConverter.cs

@@ -3,14 +3,15 @@
 using System.Reflection;
 using Newtonsoft.Json;
 using Newtonsoft.Json.Linq;
+using Contentstack.Utils.Constants;
 
 namespace Contentstack.Utils.Converters
 {
     public class RTEJsonConverter : JsonConverter
     {
         public override bool CanConvert(Type objectType)
         {
-            throw new NotImplementedException();
+            throw new InvalidOperationException(ErrorMessages.InvalidRteJson);
         }
 
         public override object ReadJson(JsonReader reader, Type objectType,