Companion to the connector's delete op (gateway-gateway 119a228). The
fresh-final unfurl route re-posts the completed reply and previously left
the sealed streamed preview behind (double delivery). delete_message now
emits op=delete when the negotiated descriptor advertises it; without the
advertisement it returns False with zero wire traffic, degrading to the
old leave-the-preview behavior against older connectors.
Consumer-level test drives placeholder -> stamped fresh final -> delete
of the original preview id.
Victor Kyriazakos
3f36c87e1e