What ConversationEngine.CreateFork needs. A null ForkFromSequence starts before the first message.
ParentBranchID must be the branch that owns the row at ForkFromSequence (null for a Main row); the
fork-point helpers (ConversationEngine.ForkPointFrom and the others) produce that.
What ConversationEngine.CreateFork needs. A null ForkFromSequence starts before the first message. ParentBranchID must be the branch that owns the row at ForkFromSequence (null for a Main row); the fork-point helpers (ConversationEngine.ForkPointFrom and the others) produce that.