Migrate existing AI Agent PMs to a new bot user after per-agent user change (#43676)

We run agent accounts across several Discourse instances and verify every re-map by comparing /u/.json ids: PMs bind to the numeric user id, not the username, which is why a renamed account keeps its inbox while a brand-new bot user starts empty. Did inviting the new bot user into an existing PM let it reply there and accept new messages from that conversation?