- fix(perf): use messagesRef.current in loadMoreMessages to avoid
recreating callback on every streamed token; messagesRef is already
kept in sync on every render so no useEffect needed — removes
`messages` from useCallback deps
- fix(api): reject invalid order query param with 400 instead of
silently ignoring; valid values are 'asc' and 'desc'
Tests:
- useChat: loadMoreMessages identity is stable when messages array changes
- chat-routes: GET /messages?order=invalid returns 400
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