fix(agent): 插话 - 独立运行时路径的送达与退回 (#673)

复现：Agent 运行中发送插话，独立 Agent 运行时（Pi）下既不送达模型也不退回用户，插话一直停在待送队列里。

原因与修法：插话送达点原本只有旧 Go 执行循环（advanceCloudAgent 开步前出队）；Pi 运行时接手执行后调度循环不再推进。现把 Pi 路径的送达搬到 /model 模型步：出队、规范历史追加与 delivered 事件同在入队的一次 CAS 写入里，响应携带 steeringMessages 交给运行时 session.steer。完成、失败、取消、审批拒绝与步数耗尽统一退回；最终回答之后的插话在预算富余时续跑一步。完成闸一次带走多条插话时，合并后的续跑提示词也记入指纹，避免被恢复闸当成遗留提示词清空。

backend/internal/app/cloud_agent_runtime_media.go

@@ -462,6 +462,7 @@ func (s *Service) DecideCloudAgentApproval(userID, id, approvalID, decision, rea
 				current.Status = "rejected"
 				current.FailureMessage = ""
 				state.Approval = nil
+				cloudAgentDropInterjections(id, "本轮已结束：已拒绝本次生成", &state)
 				state.event(id, "approval_decided", map[string]any{
 					"approvalId": approvalID,
 					"decision":   decision,
@@ -641,6 +642,7 @@ func (s *Service) CancelCloudAgent(ctx context.Context, userID, id string) error
 				current.CanvasID, current.ActiveTaskID, current.MediaTaskID = state.Request.CanvasID, state.ActiveTaskID, state.MediaTaskID
 				if firstCancellation {
 					state.event(id, "run_cancelled", map[string]any{"source": "user_request", "activeTaskId": state.ActiveTaskID, "mediaTaskId": state.MediaTaskID, "text": "用户取消接口已接收请求，正在取消关联任务"})
+					cloudAgentDropInterjections(id, "本轮已结束：用户已取消本轮", &state)
 					if saveErr := cloudAgentSave(current, &state); saveErr != nil {
 						// Cancellation must still work if the transcript is oversized.
 						log.Printf("[cloud-agent] cancellation event unavailable for run %s: checkpoint rejected", id)