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/tests/api/alerting/api_provisioning_test.go

@@ -47,13 +47,32 @@ func TestProvisioning(t *testing.T) {
 		url := fmt.Sprintf("http://%s/api/v1/provisioning/policies", grafanaListedAddr)
 		body := `
 		{
-			"receiver": "grafana-default-email",
+			"receiver": "test-receiver",
 			"group_by": [
 				"..."
 			],
 			"routes": []
 		}`
 
+		// As we check if the receiver exists that is referenced in the policy,
+		// we first need to create it, so the tests passes correctly.
+		urlReceiver := fmt.Sprintf("http://%s/api/v1/provisioning/contact-points", grafanaListedAddr)
+		bodyReceiver := `
+		{
+			"name": "test-receiver",
+			"type": "slack",
+			"settings": {
+				"recipient": "value_recipient", 
+				"token": "value_token"
+			}
+		}`
+
+		req := createTestRequest("POST", urlReceiver, "admin", bodyReceiver)
+		resp, err := http.DefaultClient.Do(req)
+		require.NoError(t, err)
+		require.NoError(t, resp.Body.Close())
+		require.Equal(t, 202, resp.StatusCode)
+
 		t.Run("un-authenticated GET should 401", func(t *testing.T) {
 			req := createTestRequest("GET", url, "", "")
 
@@ -130,7 +149,6 @@ func TestProvisioning(t *testing.T) {
 			resp, err := http.DefaultClient.Do(req)
 			require.NoError(t, err)
 			require.NoError(t, resp.Body.Close())
-
 			require.Equal(t, 202, resp.StatusCode)
 		})
 	})