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/tooling/definitions/cortex-ruler.go

@@ -257,7 +257,7 @@ type ApiRuleNode struct {
 	Record      string            `yaml:"record,omitempty" json:"record,omitempty"`
 	Alert       string            `yaml:"alert,omitempty" json:"alert,omitempty"`
 	Expr        string            `yaml:"expr" json:"expr"`
-	For         model.Duration    `yaml:"for,omitempty" json:"for,omitempty"`
+	For         *model.Duration   `yaml:"for,omitempty" json:"for,omitempty"`
 	Labels      map[string]string `yaml:"labels,omitempty" json:"labels,omitempty"`
 	Annotations map[string]string `yaml:"annotations,omitempty" json:"annotations,omitempty"`
 }

pkg/services/ngalert/api/tooling/definitions/cortex-ruler_test.go

@@ -50,7 +50,7 @@ func Test_Rule_Marshaling(t *testing.T) {
 			desc: "grafana with for, annotation and label properties",
 			input: PostableExtendedRuleNode{
 				ApiRuleNode: &ApiRuleNode{
-					For:         dur,
+					For:         &dur,
 					Annotations: map[string]string{"foo": "bar"},
 					Labels:      map[string]string{"label1": "val1"}},
 				GrafanaManagedAlert: &PostableGrafanaRule{},
@@ -136,7 +136,7 @@ func Test_Rule_Group_Marshaling(t *testing.T) {
 				Rules: []PostableExtendedRuleNode{
 					{
 						ApiRuleNode: &ApiRuleNode{
-							For:         dur,
+							For:         &dur,
 							Annotations: map[string]string{"foo": "bar"},
 							Labels:      map[string]string{"label1": "val1"},
 						},