The composer keeps a sent message, its references, and its attachments
read-only in a sending phase until session/prompt resolves, so an
admission listener can hold or refuse a prompt before it appears as sent.
The Session publishes the local echo and leaves the blank layout only
after acceptance; a refusal or failure keeps the draft, and
session/prompt-rejected with presented: true adds no toast.
This carries the client half of deepseek-harness/deepseek-harness#6178.