[MM-51517] Sentry crash fix (#22985)

* add cloud limit check

* add test

* add warning for the inaccessible root-post

---------

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

server/channels/app/post.go

@@ -1901,6 +1901,12 @@ func isCommentMention(user *model.User, post *model.Post, otherPosts map[string]
 		return mentioned
 	}
 
+	if _, ok := otherPosts[post.RootId]; !ok {
+		mlog.Warn("Can't determine the comment mentions as the rootPost is past the cloud plan's limit", mlog.String("rootPostID", post.RootId), mlog.String("commentID", post.Id))
+
+		return false
+	}
+
 	// Whether or not the user was mentioned because they started the thread
 	mentioned := otherPosts[post.RootId].UserId == user.Id