How to setup X-Forwarded-For HTTP header to preserve the original client IP address for traffic translated by a SNAT ?

just to clarify, you want to rewrite the list of IPs from the XFF header to be only the first IP in the list? If so, this should work for you.

when HTTP_REQUEST {
  HTTP::header replace X-Forwarded-For [getfield [HTTP::header X-Forwarded-For] "," 1]
}