Use #if NET to compile out error data population on .NET Framework

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/McpSessionHandler.cs

@@ -454,25 +454,20 @@ public async Task<JsonRpcResponse> SendRequestAsync(JsonRpcRequest request, Canc
             {
                 LogSendingRequestFailed(EndpointName, request.Method, error.Error.Message, error.Error.Code);
                 var exception = new McpProtocolException($"Request failed (remote): {error.Error.Message}", (McpErrorCode)error.Error.Code);
-                
+
+#if NET
                 // Populate exception.Data with the error data if present.
                 // When deserializing JSON, Data will be a JsonElement.
+                // Note: This is not supported on .NET Framework because Exception.Data uses ListDictionaryInternal
+                // which requires values to be marked with [Serializable], and JsonElement is not serializable.
                 if (error.Error.Data is JsonElement jsonElement && jsonElement.ValueKind == JsonValueKind.Object)
                 {
                     foreach (var property in jsonElement.EnumerateObject())
                     {
-                        try
-                        {
-                            exception.Data[property.Name] = property.Value;
-                        }
-                        catch (ArgumentException)
-                        {
-                            // On .NET Framework, Exception.Data uses ListDictionaryInternal which requires
-                            // values to be serializable. JsonElement is not marked as [Serializable], so
-                            // setting it throws ArgumentException. We silently skip such entries.
-                        }
+                        exception.Data[property.Name] = property.Value;
                     }
                 }
+#endif
 
                 throw exception;
             }