[MM-56266] Check for public channels when getting channel member counts to avoid 403 errors (#25831)

* [MM-56266] Check for public channels when getting channel member counts to avoid 403 errors

* Fix test

server/channels/api4/channel.go

@@ -698,11 +698,19 @@ func getChannelsMemberCount(c *Context, w http.ResponseWriter, r *http.Request)
 	}
 
 	channelIDs := model.ArrayFromJSON(r.Body)
-	if !c.App.SessionHasPermissionToChannels(c.AppContext, *c.AppContext.Session(), channelIDs, model.PermissionReadChannel) {
-		c.SetPermissionError(model.PermissionReadChannel)
+	channels, err := c.App.GetChannels(c.AppContext, channelIDs)
+	if err != nil {
+		c.Err = err
 		return
 	}
 
+	for _, channel := range channels {
+		if !c.App.HasPermissionToReadChannel(c.AppContext, c.AppContext.Session().UserId, channel) {
+			c.SetPermissionError(model.PermissionReadChannel)
+			return
+		}
+	}
+
 	channelsMemberCount, err := c.App.GetChannelsMemberCount(c.AppContext, channelIDs)
 
 	if err != nil {