Steps to reproduce the issue

When saving Proxy Settings in Joomla Global Config, no validation of the credentials takes place, the proxy is not tested to see if it works at all.

This is different to the other things in global config like db credentials, Mail credentials, Redis Session credentials etc...

Expected result

When trying to save proxy credentials and host and port - that a request is attempted through that proxy to validate the user supplied settings are correct.

Actual result

When trying to save proxy credentials and host and port - no request is attempted through that proxy to validate the user supplied settings are correct. This can allow the user to misconfigure their site with a typo and not notice. This allows the saving of untested credentials and invalid server names.