fix(ai-chat): prevent duplicate chat row on first-turn error; add adoption tests

Addresses the PR #138 review.

Blocker 1 — duplicate chat row: a brand-new chat whose first turn errors BEFORE
the SSE 'start' chunk never receives the authoritative chatId, so metadata
adoption can't run; a retry then sent chatId:null and the server inserted a
SECOND chat row, orphaning the first turn. Keep metadata adoption as the primary
path (resolveAdoptedChatId) and add a bounded, unambiguous fallback: on a
new-chat finish with no server id, snapshot the known chat ids and, once the
list refetch lands, adopt the SINGLE newly-appeared id (pickNewlyCreatedChatId).
Zero or >1 new ids (e.g. two tabs racing) → no adoption — no items[0] guessing,
so #137 stays fixed. The wait-for-refetch guard compares set membership (robust
to a concurrent delete), and the diff dedupes so a repeated id from a paginated
list never reads as ambiguous.

Blocker 2 — tests: new adopt-chat-id.test.ts covers both pure helpers (adopt
decision + newly-created-id diff incl. dedupe/reorder); the server
messageMetadata callback is extracted to chatStreamStartMetadata and unit-tested
(start -> {chatId}, otherwise undefined).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -269,9 +269,11 @@ export default function ChatThread({
     },
     // `onError` runs in addition to `onFinish` (which ai@6 also calls on error).
     // Log the raw failure here for devtools; the UI shows a friendly classified
-    // banner via `error` below. We still call `onTurnFinished()` (idempotent with
-    // the onFinish call) so a brand-new chat that fails its first turn is adopted
-    // and the chat list refreshes immediately rather than after a manual refresh.
+    // banner via `error` below. We still call `onTurnFinished()` with NO server id
+    // (idempotent with the onFinish call): for a brand-new chat that ARMS the
+    // bounded list-refetch fallback (adopt the single newly-appeared chat once the
+    // refetch lands); for an existing chat it just refreshes the chat list
+    // immediately rather than after a manual refresh.
     onError: (streamError) => {
       // Surface the raw failure in the browser console (devtools) for debugging;
       // the UI separately shows a friendly classified banner (see errorView).