Alerting: Prevent inhibition rules in Grafana Alertmanager (#81712)

This commit prevents saving configurations containing inhibition
rules in Grafana Alertmanager. It does not reject inhibition
rules when using external Alertmanagers, such as Mimir. This meant
the validation had to be put in the MultiOrgAlertmanager instead of
in the validation of PostableUserConfig. We can remove this when
inhibition rules are supported in Grafana Managed Alerts.

pkg/tests/api/alerting/testing.go

@@ -3,6 +3,7 @@ package alerting
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -341,6 +342,39 @@ func (a apiClient) UpdateAlertRuleOrgQuota(t *testing.T, orgID int64, limit int6
 	assert.Equal(t, http.StatusOK, resp.StatusCode)
 }
 
+func (a apiClient) PostConfiguration(t *testing.T, c apimodels.PostableUserConfig) (bool, error) {
+	t.Helper()
+
+	b, err := json.Marshal(c)
+	require.NoError(t, err)
+
+	u := fmt.Sprintf("%s/api/alertmanager/grafana/config/api/v1/alerts", a.url)
+	req, err := http.NewRequest(http.MethodPost, u, bytes.NewReader(b))
+	require.NoError(t, err)
+	req.Header.Set("Content-Type", "application/json")
+	client := &http.Client{}
+	resp, err := client.Do(req)
+	require.NoError(t, err)
+	require.NotNil(t, resp)
+
+	defer func() {
+		_ = resp.Body.Close()
+	}()
+	b, err = io.ReadAll(resp.Body)
+	require.NoError(t, err)
+
+	data := struct {
+		Message string `json:"message"`
+	}{}
+	require.NoError(t, json.Unmarshal(b, &data))
+
+	if resp.StatusCode == http.StatusAccepted {
+		return true, nil
+	}
+
+	return false, errors.New(data.Message)
+}
+
 func (a apiClient) PostRulesGroupWithStatus(t *testing.T, folder string, group *apimodels.PostableRuleGroupConfig) (apimodels.UpdateRuleGroupResponse, int, string) {
 	t.Helper()
 	buf := bytes.Buffer{}