refactor(ai-chat): extract useChatSession hook + lock the id lifecycle with tests

Addresses the 2nd PR #138 review (test debt + the Variant-B architecture ask).

The new→persisted chat id lifecycle (mount key, both adoption paths, the
history-load latch, the render-phase reconciler, onTurnFinished) is moved out of
the 768-line window into a new useChatSession hook driven by a pure
threadSessionReducer (reconcile/adopt), so adopt-vs-switch is one explicit
dispatch point and the scattering the review flagged is gone (window: 768→~620).

Tests (the blockers):
- use-chat-session.test.tsx — hook-level locks incl. the #137 regression
  (adopts the authoritative streamed id 'A', NOT chats.items[0]='B' — fails on
  the old heuristic), the error-path fallback (arm/adopt/ambiguous/add+delete),
  the disarm-on-reconcile lock (a fallback armed then switched away must not be
  adopted by a late refetch), in-place-adopt-keeps-key vs external-switch-remount,
  and the waitingForHistory latch.
- extractServerChatId (reading message.metadata.chatId) and newlyAddedChatIds
  extracted as pure helpers with unit tests; threadSessionReducer tested.

Cleanups: single canonical #137 explanation in adopt-chat-id.ts (other sites
reference it); fallback effect computes the set diff once; invalidate callbacks
memoized; redundant invariant tests folded.

Behavior preserved — re-verified live (z.ai glm-5.2): new-chat adopt + 2nd turn
in the same row, no mid-conversation remount, two-tab race leak-free, switch to
an existing chat reseeds full history, reload restores history.

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

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

@@ -22,6 +22,7 @@ import {
   IAiRole,
 } from "@/features/ai-chat/types/ai-chat.types.ts";
 import { describeChatError } from "@/features/ai-chat/utils/error-message.ts";
+import { extractServerChatId } from "@/features/ai-chat/utils/adopt-chat-id.ts";
 import {
   dequeue,
   enqueueMessage,
@@ -57,12 +58,10 @@ interface ChatThreadProps {
   /** Display name for the assistant label / typing line (the role name);
    *  forwarded to MessageList. Absent => the generic "AI agent". */
   assistantName?: string;
-  /** Called when a turn finishes; the parent refreshes the chat list and, for
-   *  a new chat, adopts the freshly created chat id. `serverChatId` is the
-   *  authoritative id the server attached to the streamed assistant message
-   *  metadata (see the server's `messageMetadata`); the parent adopts THIS for a
-   *  new chat instead of guessing the newest chat in the list (fixes the two-tab
-   *  adoption race, #137). Undefined on a failed turn that produced no metadata. */
+  /** Called when a turn finishes; the parent refreshes the chat list and, for a
+   *  new chat, adopts the freshly created chat id. `serverChatId` is the
+   *  authoritative id the server streamed on the assistant message metadata, or
+   *  undefined on a failed turn — see adopt-chat-id.ts for the full #137 design. */
   onTurnFinished: (serverChatId?: string) => void;
   /** Parent-owned ref that this thread keeps updated with its live useChat
    *  snapshot (full message list + streaming flag), so the header's
@@ -251,13 +250,10 @@ export default function ChatThread({
     // would be wrong, so on Stop/disconnect/error the queue is left intact for
     // the user to decide.
     onFinish: ({ message, isAbort, isDisconnect, isError }) => {
-      // The server attaches the authoritative chatId to the streamed assistant
-      // message metadata (see `messageMetadata` in ai-chat.service.ts). Forward it
-      // so the parent adopts the REAL created chat id for a new chat, rather than
-      // guessing the newest chat in the list (which races a second tab — #137).
-      const serverChatId = (message?.metadata as { chatId?: string } | undefined)
-        ?.chatId;
-      onTurnFinished(serverChatId);
+      // Forward the authoritative server chatId (streamed on the assistant
+      // message metadata) so the parent adopts the REAL created chat id for a new
+      // chat — see adopt-chat-id.ts for the full #137 design.
+      onTurnFinished(extractServerChatId(message));
       // Show a neutral "stopped" marker for an aborted turn; the red error banner
       // (via `error`) already covers isError, and a clean finish clears any marker.
       if (isError) setStopNotice(null);