Alerting: validate that the receiver exist when updating routing tree (#51561) (#51758)

* Alerting: validate that the receiver exist when updating routing tree

* rename interface

* add missing file

* change constructor

* fix e2e tests

* only import package once

* add unit test for bug

* wording

* close response body

* Update pkg/services/ngalert/api/tooling/definitions/alertmanager_validation.go

* refactor to remove database roundtrip

(cherry picked from commit e64cde8727)

Co-authored-by: Jean-Philippe Quéméner <JohnnyQQQQ@users.noreply.github.com>

pkg/services/ngalert/api/tooling/definitions/alertmanager_validation.go

@@ -101,6 +101,19 @@ func (r *Route) Validate() error {
 	return r.validateChild()
 }
 
+func (r *Route) ValidateReceivers(receivers map[string]struct{}) error {
+	if _, exists := receivers[r.Receiver]; !exists {
+		return fmt.Errorf("receiver '%s' does not exist", r.Receiver)
+	}
+	for _, children := range r.Routes {
+		err := children.ValidateReceivers(receivers)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 func (mt *MuteTimeInterval) Validate() error {
 	s, err := yaml.Marshal(mt.MuteTimeInterval)
 	if err != nil {