Alerting: Fix misleading status code in provisioning API (#67331)

Fixes #66249

pkg/services/ngalert/api/api_provisioning.go

@@ -266,6 +266,9 @@ func (srv *ProvisioningSrv) RouteGetAlertRules(c *contextmodel.ReqContext) respo
 func (srv *ProvisioningSrv) RouteRouteGetAlertRule(c *contextmodel.ReqContext, UID string) response.Response {
 	rule, provenace, err := srv.alertRules.GetAlertRule(c.Req.Context(), c.OrgID, UID)
 	if err != nil {
+		if errors.Is(err, alerting_models.ErrAlertRuleNotFound) {
+			return response.Empty(http.StatusNotFound)
+		}
 		return ErrResp(http.StatusInternalServerError, err, "")
 	}
 	return response.JSON(http.StatusOK, ProvisionedAlertRuleFromAlertRule(rule, provenace))

pkg/services/ngalert/api/api_provisioning_test.go

@@ -307,6 +307,14 @@ func TestProvisioningApi(t *testing.T) {
 			require.Equal(t, 404, response.Status())
 		})
 
+		t.Run("are missing, GET returns 404", func(t *testing.T) {
+			sut := createProvisioningSrvSut(t)
+			rc := createTestRequestCtx()
+			response := sut.RouteRouteGetAlertRule(&rc, "does not exist")
+
+			require.Equal(t, 404, response.Status())
+		})
+
 		t.Run("have reached the rule quota, POST returns 403", func(t *testing.T) {
 			env := createTestEnv(t)
 			quotas := provisioning.MockQuotaChecker{}