list_thread_messages matched event_type == "ai_message" to find each
run's last AI message, but RunJournal stores AI messages as
"llm.ai.response" and the event store returns that verbatim. No code
writes "ai_message", so the match never hit: feedback was never attached
(every message returned feedback=null) and the grouped-feedback query ran
on every request for nothing.
Match "llm.ai.response", and only run the grouped-feedback query when the
thread actually has an AI message to attach it to. Adds a regression test
for the per-run attachment and the no-AI-message lazy-query path.
Fixes#3650.
