Test send through a Provider

Sends one real message through a stored Email or SMS Provider to validate its credentials. Two failure outcomes are distinguished: 422 IDE-1016 means the provider rejected the credentials (fatal — fix them); 400 IDE-1017 means the message was not delivered but no credential failure was reported, which is what a Twilio trial account returns for an unverified recipient even with valid credentials. Both outcomes append the upstream provider’s own reason to the response message, with the recipient and any other address or phone number removed. A 500 means the send never reached the provider at all and says nothing about its configuration. Not supported for OAuth providers (422 IDE-1018).