MM-65575: Fix server panic when bot posts trigger persistent notifications (#34174) (#34778)

* reproduce panic with test

* allow bots in the profile map

* explicitly prevent sending notifications to bots

* persistent notifications: handle senders not in the channel

server/channels/app/post_persistent_notification.go

@@ -171,6 +171,18 @@ func (a *App) forEachPersistentNotificationPost(posts []*model.Post, fn func(pos
 		}
 		profileMap := channelProfileMap[channel.Id]
 
+		// Ensure the sender is always in the profile map: for example, system admins can post
+		// without being a member.
+		if _, ok := profileMap[post.UserId]; !ok {
+			var sender *model.User
+			sender, err = a.Srv().Store().User().Get(context.Background(), post.UserId)
+			if err != nil {
+				return errors.Wrapf(err, "failed to get profile for sender user %s for post %s", post.UserId, post.Id)
+			}
+
+			profileMap[post.UserId] = sender
+		}
+
 		mentions := &MentionResults{}
 		// In DMs, only the "other" user can be mentioned
 		if channel.Type == model.ChannelTypeDirect {
@@ -229,15 +241,12 @@ func (a *App) persistentNotificationsAuxiliaryData(channelsMap map[string]*model
 		}
 
 		channelKeywords[c.Id] = make(MentionKeywords, len(profileMap))
-		validProfileMap := make(map[string]*model.User, len(profileMap))
 		for userID, user := range profileMap {
-			if user.IsBot {
-				continue
+			if !user.IsBot {
+				channelKeywords[c.Id].AddUserKeyword(userID, "@"+user.Username)
 			}
-			validProfileMap[userID] = user
-			channelKeywords[c.Id].AddUserKeyword(userID, "@"+user.Username)
 		}
-		channelProfileMap[c.Id] = validProfileMap
+		channelProfileMap[c.Id] = profileMap
 	}
 	return channelGroupMap, channelProfileMap, channelKeywords, channelNotifyProps, nil
 }