FailFast in UnknownMessageSerializer.Serialize (#50704)

Author: Youssef1313

src/Cli/dotnet/Commands/Test/MTP/IPC/Serializers/UnknownMessageSerializer.cs

@@ -17,7 +17,5 @@ public object Deserialize(Stream _)
     }
 
     public void Serialize(object _, Stream stream)
-    {
-        WriteInt(stream, Id);
-    }
+        => Environment.FailFast("SDK never serializes UnknownMessage. This should never be hit.");
 }