[MM-27622] Publish messages as SystemBot when a user is required (#17598)

Automatic Merge

model/bot.go

@@ -17,6 +17,7 @@ const (
 	BOT_DESCRIPTION_MAX_RUNES    = 1024
 	BOT_CREATOR_ID_MAX_RUNES     = KEY_VALUE_PLUGIN_ID_MAX_RUNES // UserId or PluginId
 	BOT_WARN_METRIC_BOT_USERNAME = "mattermost-advisor"
+	BOT_SYSTEM_BOT_USERNAME      = "system-bot"
 )
 
 // Bot is a special type of User meant for programmatic interactions.