A Telegram webhook passed Test while every real alert was rejected with
"message text is empty" (#2540). The Test path always overlaid the
service template's headers, but real deliveries sent the stored header
copy that the UI seeded at creation. An edit can leave that copy blank,
masked or replaced. Telegram reads arguments from a body only when it is
labelled as JSON or form data, and checks text before chat_id, so a JSON
body without a JSON Content-Type produces exactly that error.
When a built-in service renders its own body (no custom template), the
template's static headers describe that body. Apply them on every real
firing and recovery through prepareWebhookDeliveryContext, replace stored
values under any spelling of the name, and route the Test path through
the same helper so Test predicts real delivery. Custom templates and
generic webhooks keep their stored headers.
rcourtman
06cdaa577a