fix(AGC): 目录缺 unavailableModels 时不再崩溃

- applyCatalog 归一化 catalog.unavailableModels，修掉按 reason 取降级文案时对 undefined 解引用（此前 26 个用例挂在这）
- 用例跟进新降级文案；disabled 用例补上下线桶，断言「该模型已下线」专用文案

apps/ai-game-creator-shell/src/features/project-workspace/ConversationModelSelect.tsx

@@ -144,6 +144,9 @@ export function ConversationModelSelect({
       showBusy: boolean,
       epochAtRequest: number,
     ) => {
+      // 目录契约的 `unavailableModels` 是加法字段：旧缓存 / 局部数据可能缺失。
+      // 统一归一化一次，避免下面 setState 与按 reason 取文案时解引用 undefined。
+      const catalogUnavailableModels = catalog.unavailableModels ?? [];
       if (
         mountedRef.current &&
         (appliedRevisionRef.current !== catalog.revision ||
@@ -151,7 +154,7 @@ export function ConversationModelSelect({
       ) {
         appliedRevisionRef.current = catalog.revision;
         setModels(catalog.models);
-        setUnavailableModels(catalog.unavailableModels);
+        setUnavailableModels(catalogUnavailableModels);
         setDefaultModelId(catalog.defaultModelId);
       }
       const invoke = resolveTauriInvoke();
@@ -190,7 +193,7 @@ export function ConversationModelSelect({
         if (saved) {
           // 命中不可用桶时按 reason 给具体原因；两桶都没有（后台直接删除）走泛化文案。
           nextNotice = unavailableModelFallbackNotice(
-            catalog.unavailableModels.find((model) => model.id === saved)
+            catalogUnavailableModels.find((model) => model.id === saved)
               ?.reason,
           );
         }

apps/ai-game-creator-shell/tests/conversationModelSelect.test.tsx

@@ -315,7 +315,7 @@ test('falls back to the default model when the saved selection was removed', asy
   }));
   const onReady = vi.fn();
   render(<ConversationModelSelect disabled={false} onReady={onReady} />);
-  await screen.findByText('所选模型已停用，已切换为默认模型');
+  await screen.findByText('所选模型已不可用，已切回默认模型');
   await waitFor(() => expect(onReady).toHaveBeenLastCalledWith(true));
   expect(screen.queryByText('private-old-model')).toBeNull();
   expect(invoke).toHaveBeenCalledWith('select_game_creator_model', {
@@ -521,12 +521,22 @@ test('pre-send validation falls back when the selected model is disabled', async
   vi.mocked(loadClientLlmModels).mockResolvedValue({
     defaultModelId: 'quality',
     models: [{ id: 'quality', displayName: '高质量', agentMode: 'codex' }],
+    // `fast` 仍在下线桶里：降级文案要按 `disabled` 取「该模型已下线」。
+    unavailableModels: [
+      {
+        id: 'fast',
+        displayName: '快速',
+        agentMode: 'cc',
+        protocol: 'anthropic',
+        reason: 'disabled',
+      },
+    ],
     revision: 2,
   });
   await act(async () => {
     await expect(ref.current?.ensureUsable()).resolves.toBe(true);
   });
-  expect(screen.getByText('所选模型已停用，已切换为默认模型')).not.toBeNull();
+  expect(screen.getByText('所选模型已下线，已切换为默认模型')).not.toBeNull();
   expect(savedModelId).toBe('quality');
   expect(invoke).toHaveBeenCalledWith('select_game_creator_model', {
     modelId: 'quality',