Edit the body of an unsent message draft. Works for every draft in the sequence: the first outreach message, follow-ups, and re-engagements alike. A draft stays editable while it is pending approval and after it has been approved but not yet sent (the send path reads the latest body at send time), so an approved draft can still be corrected before its send window. Once a message has been sent it can no longer be edited. An already-sent message returns 400. A 409 is reserved for the narrow case where the message is sent concurrently, after this request read its status but before the write lands. A draft stays editable after its campaign is deleted, matching the in-app behaviour. subject is accepted on INMAIL messages only, and cannot be blank once set. Character limits are enforced per message type: CONNECT 300 (200 on a Basic, non-Premium LinkedIn seat), INMAIL 1700 (subject 200), FOLLOW_UP 1000, RE_ENGAGEMENT 1000, FIRST_DEGREE_INTRO 8000.