Alerting: Support JSON Content-Type in the Prometheus conversion API (#102627)

Alerting: Support content-type: json in conversion API

pkg/tests/api/alerting/testing.go

@@ -1145,16 +1145,26 @@ func (a apiClient) RawConvertPrometheusPostRuleGroup(t *testing.T, namespaceTitl
 		path = "%s/api/convert/api/prom/rules/%s"
 	}
 
-	data, err := yaml.Marshal(promGroup)
-	require.NoError(t, err)
+	// Based on the content-type header, marshal the data to JSON or YAML
+	contentType := headers["Content-Type"]
+	var data []byte
+	var err error
+	if contentType == "application/json" {
+		data, err = json.Marshal(promGroup)
+		require.NoError(t, err)
+	} else {
+		data, err = yaml.Marshal(promGroup)
+		require.NoError(t, err)
+	}
+
 	buf := bytes.NewReader(data)
 
 	req, err := http.NewRequest(http.MethodPost, fmt.Sprintf(path, a.url, namespaceTitle), buf)
 	require.NoError(t, err)
-	req.Header.Add("X-Grafana-Alerting-Datasource-UID", datasourceUID)
+	req.Header.Set("X-Grafana-Alerting-Datasource-UID", datasourceUID)
 
 	for key, value := range headers {
-		req.Header.Add(key, value)
+		req.Header.Set(key, value)
 	}
 
 	return sendRequestJSON[apimodels.ConvertPrometheusResponse](t, req, http.StatusAccepted)