Alerting: Fix rule API to accept 0 duration of field For (#50992)

* make 'for' pointer to distinguish between missing field and 0
* set 'for' to -1 if the value is missing but not allow negative in the request + path -1 with the value from original rule
* update store validation to not allow negative 'for'
* update usages to use pointer

pkg/services/ngalert/api/api_ruler.go

@@ -484,8 +484,9 @@ func toGettableExtendedRuleNode(r ngmodels.AlertRule, namespaceID int64, provena
 			Provenance:      provenance,
 		},
 	}
+	forDuration := model.Duration(r.For)
 	gettableExtendedRuleNode.ApiRuleNode = &apimodels.ApiRuleNode{
-		For:         model.Duration(r.For),
+		For:         &forDuration,
 		Annotations: r.Annotations,
 		Labels:      r.Labels,
 	}