feat(trans_process): conditionally set NextCronTime to prevent overwriting

Set NextCronTime only when it is nil or zero to preserve existing
scheduled times and prevent unintended cron interval resets during
transaction processing.
只有当没有提供NextCronTime时才初始化，否则使用提供的进行saveNew)

dtmsvr/trans_process.go

@@ -77,7 +77,9 @@ func (t *TransGlobal) processInner(ctx context.Context, branches []TransBranch)
 
 func (t *TransGlobal) saveNew() ([]TransBranch, error) {
 	t.NextCronInterval = t.getNextCronInterval(cronReset)
-	t.NextCronTime = dtmutil.GetNextTime(t.NextCronInterval)
+	if t.NextCronTime == nil || t.NextCronTime.IsZero() {
+		t.NextCronTime = dtmutil.GetNextTime(t.NextCronInterval)
+	}
 	t.ExtData = dtmimp.MustMarshalString(t.Ext)
 	if t.ExtData == "{}" {
 		t.ExtData = ""