fix: return http 204 on test notification (#16651)

This PR changes the API response for `/api/v2/notifications/test`
endpoint to HTTP 204 / No Content.

coderd/notifications.go

@@ -210,7 +210,7 @@ func (api *API) postTestNotification(rw http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	httpapi.Write(ctx, rw, http.StatusOK, nil)
+	rw.WriteHeader(http.StatusNoContent)
 }
 
 // @Summary Get user notification preferences

codersdk/notifications.go

@@ -200,10 +200,9 @@ func (c *Client) PostTestNotification(ctx context.Context) error {
 	}
 	defer res.Body.Close()
 
-	if res.StatusCode != http.StatusOK {
+	if res.StatusCode != http.StatusNoContent {
 		return ReadBodyAsError(res)
 	}
-
 	return nil
 }