Retrieve Calendar Event Message

Returns everything needed to send the event's message: the stored settings and variants (one variant when there is no split) with their ad slot assignments, the resolved values a build would use after defaults are applied (template, senders, subject, campaign pages, segments, translations, send time), the missing list of what still blocks a build (each entry has a path, a reason code, and a message), and the lifecycle status. The content_type is fixed by the event's channel and category: SMS events send an SMS, and email events send an article (Update) or a lift letter (Dedicated Offer). The settings are created on first read.