fix(ai-chat): keep provider errors visible after a new-chat remount

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.

apps/client/src/features/ai-chat/components/chat-thread.tsx

@@ -8,6 +8,7 @@ import { DefaultChatTransport } from "ai";
 import MessageList from "@/features/ai-chat/components/message-list.tsx";
 import ChatInput from "@/features/ai-chat/components/chat-input.tsx";
 import { IAiChatMessageRow } from "@/features/ai-chat/types/ai-chat.types.ts";
+import { describeChatError } from "@/features/ai-chat/utils/error-message.ts";
 import classes from "@/features/ai-chat/components/ai-chat.module.css";
 
 /** The page the user is currently viewing, sent as chat context. */
@@ -40,7 +41,15 @@ function rowToUiMessage(row: IAiChatMessageRow): UIMessage {
     Array.isArray(row.metadata?.parts) && row.metadata.parts.length > 0
       ? row.metadata.parts
       : ([{ type: "text", text: row.content ?? "" }] as UIMessage["parts"]);
-  return { id: row.id, role, parts } as UIMessage;
+  const error = row.metadata?.error;
+  return {
+    id: row.id,
+    role,
+    parts,
+    // Carry a persisted turn error so MessageItem can render it after a remount
+    // (e.g. when a new chat adopts its id) and in reopened chat history.
+    ...(error ? { metadata: { error } } : {}),
+  } as UIMessage;
 }
 
 /**
@@ -141,7 +150,7 @@ export default function ChatThread({
           mb="xs"
           title={t("Something went wrong")}
         >
-          {describeError(error, t)}
+          {describeChatError(error.message ?? "", t)}
         </Alert>
       )}
 
@@ -155,49 +164,3 @@ export default function ChatThread({
     </Box>
   );
 }
-
-/**
- * Turn a useChat error into a friendly inline message. The transport throws on
- * non-2xx with the response text/status in the message, and stream failures
- * arrive as `"<status>: <message>"` (forwarded by the server's
- * pipeUIMessageStreamToResponse onError). We keep the friendly mappings for the
- * gating responses (403 chat disabled, 503 provider not configured) and
- * otherwise surface the real provider message (e.g. 402 insufficient credits /
- * 429 rate limit) so the actual cause is visible — never a crash.
- */
-function describeError(
-  error: Error,
-  t: (key: string) => string,
-): string {
-  const msg = error.message ?? "";
-  // Our own gating responses arrive PRE-stream as the raw JSON error body
-  // (NestJS default exception shape — no global filter overrides it), which
-  // carries a numeric "statusCode" field. Match that field, NOT a bare
-  // substring, so a provider stream error that merely contains "403"/"503" (or
-  // a word like "disabled") is never misclassified and its real cause hidden.
-  if (/"statusCode"\s*:\s*403\b/.test(msg)) {
-    return t("AI chat is disabled for this workspace.");
-  }
-  if (/"statusCode"\s*:\s*503\b/.test(msg)) {
-    return t("The AI provider is not configured. Ask an administrator to set it up.");
-  }
-  // Any other failure — including provider stream failures forwarded as
-  // "<status>: <message>" (402 credits, 429 rate limit, ...) — is surfaced
-  // verbatim so the real cause is visible. Fall back to a generic message only
-  // when there is no usable text or just an opaque placeholder.
-  return providerDetail(msg) ?? t("The AI agent could not respond. Please try again.");
-}
-
-/**
- * Extract a human-readable provider detail from a useChat error message, or
- * null when there is nothing useful to show. Returns null for empty strings,
- * the AI SDK's opaque "An error occurred." placeholder, and our own post-hijack
- * "Internal server error" fallback, so the caller can use a friendly message.
- */
-function providerDetail(msg: string): string | null {
-  const trimmed = msg.trim();
-  if (!trimmed) return null;
-  if (/^an error occurred\.?$/i.test(trimmed)) return null;
-  if (/internal server error/i.test(trimmed)) return null;
-  return trimmed;
-}