MM-54778 Fix mark as unread on GMs (#24880)

* Fix mark as unread on GMs

* Don't count own messages in gms when marking as unread

* Change argument name

* Rename userId

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/post.go

@@ -1795,7 +1795,7 @@ func (a *App) countThreadMentions(c request.CTX, user *model.User, post *model.P
 
 	count := 0
 
-	if channel.Type == model.ChannelTypeDirect {
+	if channel.Type == model.ChannelTypeDirect || channel.Type == model.ChannelTypeGroup {
 		// In a DM channel, every post made by the other user is a mention
 		otherId := channel.GetOtherUserIdForDM(user.Id)
 		for _, p := range posts {
@@ -1842,16 +1842,16 @@ func (a *App) countMentionsFromPost(c request.CTX, user *model.User, post *model
 		return 0, 0, 0, err
 	}
 
-	if channel.Type == model.ChannelTypeDirect {
+	if channel.Type == model.ChannelTypeDirect || channel.Type == model.ChannelTypeGroup {
 		// In a DM channel, every post made by the other user is a mention
-		count, countRoot, nErr := a.Srv().Store().Channel().CountPostsAfter(post.ChannelId, post.CreateAt-1, channel.GetOtherUserIdForDM(user.Id))
+		count, countRoot, nErr := a.Srv().Store().Channel().CountPostsAfter(post.ChannelId, post.CreateAt-1, user.Id)
 		if nErr != nil {
 			return 0, 0, 0, model.NewAppError("countMentionsFromPost", "app.channel.count_posts_since.app_error", nil, "", http.StatusInternalServerError).Wrap(nErr)
 		}
 
 		var urgentCount int
 		if a.IsPostPriorityEnabled() {
-			urgentCount, nErr = a.Srv().Store().Channel().CountUrgentPostsAfter(post.ChannelId, post.CreateAt-1, channel.GetOtherUserIdForDM(user.Id))
+			urgentCount, nErr = a.Srv().Store().Channel().CountUrgentPostsAfter(post.ChannelId, post.CreateAt-1, user.Id)
 			if nErr != nil {
 				return 0, 0, 0, model.NewAppError("countMentionsFromPost", "app.channel.count_urgent_posts_since.app_error", nil, "", http.StatusInternalServerError).Wrap(nErr)
 			}