A/B comparison votes now reach the history

Fixed a bug where A/B comparison votes were silently discarded. The vote buttons sent display labels the endpoint did not accept, and the interface treated the rejection as success — the confirmation appeared, but the comparison history stayed empty and only ties were ever recorded. Votes are now normalized at the API boundary and stored, and a vote that genuinely fails reverts the interface and shows an error instead of a false confirmation.

Separately, sending a single A/B question on a new conversation no longer creates two sidebar conversations; both model streams now share one conversation.

No action required. New votes are recorded correctly; votes lost before this fix were never stored and cannot be recovered.