Optimize union initialization by avoiding object initializer

Changed from `union = new Union { JsonRpc = string.Empty }` to
`union = default; union.JsonRpc = string.Empty;` to avoid the
overhead of object initializer syntax and make direct assignment
to the struct field.

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

@@ -156,10 +156,8 @@ public override void Write(Utf8JsonWriter writer, JsonRpcMessage value, JsonSeri
         /// </summary>
         private static void ParseUnion(ref Utf8JsonReader reader, JsonSerializerOptions options, out Union union)
         {
-            union = new Union
-            {
-                JsonRpc = string.Empty // Initialize to avoid null reference warnings
-            };
+            union = default;
+            union.JsonRpc = string.Empty; // Initialize to avoid null reference warnings
 
             if (reader.TokenType != JsonTokenType.StartObject)
             {