Use out parameter for ParseUnion to avoid struct copy

Changed ParseUnion to return the Union struct via an out parameter
instead of by value, avoiding unnecessary copying of the large struct.

Co-authored-by: Scooletz <scooletz@users.noreply.github.com>
Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Protocol/JsonRpcMessage.cs

@@ -77,7 +77,7 @@ public sealed class Converter : JsonConverter<JsonRpcMessage>
         /// <inheritdoc/>
         public override JsonRpcMessage? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
-            var union = ParseUnion(ref reader, options);
+            ParseUnion(ref reader, options, out Union union);
 
             // All JSON-RPC messages must have a jsonrpc property with value "2.0"
             if (union.JsonRpc != JsonRpcVersion)
@@ -164,9 +164,9 @@ public override void Write(Utf8JsonWriter writer, JsonRpcMessage value, JsonSeri
         /// <summary>
         /// Manually parses a JSON-RPC message from the reader into the Union struct.
         /// </summary>
-        private static Union ParseUnion(ref Utf8JsonReader reader, JsonSerializerOptions options)
+        private static void ParseUnion(ref Utf8JsonReader reader, JsonSerializerOptions options, out Union union)
         {
-            var union = new Union
+            union = new Union
             {
                 JsonRpc = string.Empty // Initialize to avoid null reference warnings
             };
@@ -228,8 +228,6 @@ private static Union ParseUnion(ref Utf8JsonReader reader, JsonSerializerOptions
                         break;
                 }
             }
-
-            return union;
         }
 
         /// <summary>