fix: conditionally import test models to resolve Node.js module errors in production (#1163)

lib/ai/providers.ts

@@ -4,23 +4,25 @@ import {
   wrapLanguageModel,
 } from 'ai';
 import { gateway } from '@ai-sdk/gateway';
-import {
+import { isTestEnvironment } from '../constants';
+
+export const myProvider = isTestEnvironment
+  ? (() => {
+      const {
         artifactModel,
         chatModel,
         reasoningModel,
         titleModel,
-} from './models.test';
+      } = require('./models.test');
-import { isTestEnvironment } from '../constants';
+      return customProvider({
-
-export const myProvider = isTestEnvironment
-  ? customProvider({
         languageModels: {
           'chat-model': chatModel,
           'chat-model-reasoning': reasoningModel,
           'title-model': titleModel,
           'artifact-model': artifactModel,
         },
-    })
+      });
+    })()
   : customProvider({
       languageModels: {
         'chat-model': gateway.languageModel('xai/grok-2-vision-1212'),