task/leave
Self-only: caller removes themselves fromtask_participants AND
every conversation_participants row under the task. See spec
body Goal 2 for the atomicity, idempotency, and
last-participant-task-closure contract.
Notification emission for each conversation the caller leaves uses
TaskConversationParticipantsRemovedNotificationDefinition with
reason: "task_leave". If removal empties task_participants
the task transitions to status = 'closed' and
TaskClosedNotificationDefinition fires alongside in the same
transaction.
Parameters
Branded TaskId