* fix(flows): persist custom conversational replies after fallback append
Custom @listen routes that only return a public string were appended after kickoff, so @persist snapshots missed the assistant turn. Snapshot again from the same persist path once the fallback writes state.messages.
* test(flows): cover @persist restore of custom conversational replies
Add regression coverage for custom @listen returns across fresh Flow instances, no double-append on built-in converse, and a single user/assistant message-added event.
* docs: note that public listen returns persist as assistant replies
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