Surfacing the stream error via useChat().error alone was not enough: on a
brand-new chat the errored turn still fires onFinish -> onTurnFinished, which
adopts the freshly-created chat id and changes the <ChatThread> key, remounting
it with a fresh useChat whose transient `error` is gone. The thread re-seeds
from persisted history, where the assistant row has empty parts and the error
lives only in metadata.error — which was never rendered. Result: an empty
"AI agent" row and no visible error.
- Render the persisted metadata.error inline in MessageItem, so the error
survives the remount and is also shown in reopened chat history.
- Carry metadata.error onto the rebuilt UIMessage in rowToUiMessage.
- Extract the error formatter into utils/error-message.ts (describeChatError)
and reuse it for both the live Alert and the persisted error.
- Add metadata.error to the IAiChatMessageRow type.
Client-only; the server already persists metadata.error. No new i18n keys.
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