Added check for invalid json in JsonSocketMessageHandler and virtual GetTypeIdentifierNonJson for handling non-json messages

CryptoExchange.Net/Clients/RestApiClient.cs

@@ -439,6 +439,10 @@ namespace CryptoExchange.Net.Clients
                 var outputOriginalData = ApiOptions.OutputOriginalData ?? ClientOptions.OutputOriginalData;
                 if (outputOriginalData || MessageHandler.RequiresSeekableStream || !response.IsSuccessStatusCode)
                 {
+                    // Create a seekable stream from the response stream if:
+                    // 1. We need to output the original data
+                    // 2. The message handler requires a seekable stream
+                    // 3. The response indicates error and we want to output (part of) the returned data
                     responseStream = await CopyStreamAsync(responseStream).ConfigureAwait(false);
                     using var reader = new StreamReader(responseStream, Encoding.UTF8, false, 4096, true);
                     if (outputOriginalData)

CryptoExchange.Net/Converters/SystemTextJson/MessageHandlers/JsonSocketMessageHandler.cs

@@ -165,6 +165,14 @@ namespace CryptoExchange.Net.Converters.SystemTextJson.MessageHandlers
             return null;
         }
 
+        /// <summary>
+        /// Return type identifier for non-json messages
+        /// </summary>
+        protected virtual string? GetTypeIdentifierNonJson(ReadOnlySpan<byte> data, WebSocketMessageType? webSocketMessageType)
+        {
+            return null;
+        }
+
         /// <inheritdoc />
         public virtual string? GetTypeIdentifier(ReadOnlySpan<byte> data, WebSocketMessageType? webSocketMessageType)
         {
@@ -173,6 +181,12 @@ namespace CryptoExchange.Net.Converters.SystemTextJson.MessageHandlers
             int? arrayIndex = null;
 
             _searchResult.Clear();
+            if (data[0] != 0x5B && data[0] != 0x7B)
+            {
+                // Message doesn't start with `{` or `[`, not valid for processing as json
+                return GetTypeIdentifierNonJson(data, webSocketMessageType);
+            }
+
             var reader = new Utf8JsonReader(data);
             while (reader.Read())
             {