Reliability fixes in SaferPayClient

[clementfavre]

Hi, bundled four small things I noticed in SaferPayClient.cs.

• The Dispose method calls itself in its own body, so any consumer doing using var client = new SaferPayClient(...) crashes with a StackOverflowException at end of scope. The class doesn't hold anything to release, so I just emptied it
• The 4-arg constructor never assigns _jsonSerializerSettings, while the other two do. It happens to be the one Settings.Client() and every Channel shortcut use, so most consumers end up with a null value and Newtonsoft silently falls back to its own defaults. I chained it to the existing 2-arg ctor
• All the Send, Get, Post and Delete methods create a RestClient without ever disposing it. Since RestSharp wraps an HttpClient, that leaks sockets under load. I added using on each one
  And in the catch blocks of SendAsync, two throw spex / throw ex statements were resetting the stack trace, so I changed them to plain throw

[viwodev]

Thanks a lot for the detailed PR and explanations.

All of the reported issues made sense and were reproducible on my side as well. The fixes around Dispose(), serializer initialization, RestClient disposal and stack trace preservation have now been merged into main, along with a few additional cleanup and reliability improvements afterward.

Really appreciate you taking the time to investigate and contribute 🚀