Fix SourceConfigParam url parameter serialization

Author: flobernd

src/Elastic.Clients.Elasticsearch.Shared/Types/Core/Search/SourceConfigParam.cs

@@ -0,0 +1,21 @@
+using System;
+
+using Elastic.Transport;
+
+#if ELASTICSEARCH_SERVERLESS
+namespace Elastic.Clients.Elasticsearch.Serverless.Core.Search;
+#else
+namespace Elastic.Clients.Elasticsearch.Core.Search;
+#endif
+
+public partial class SourceConfigParam :
+	IUrlParameter
+{
+	public string GetString(ITransportConfiguration settings) =>
+		Tag switch
+		{
+			0 => UrlFormatter.CreateString(Item1, settings)!,
+			1 => UrlFormatter.CreateString(Item2, settings)!,
+			_ => throw new InvalidOperationException()
+		};
+}