fix: replace test models with lightweight mocks to resolve module errors (#1164)

lib/ai/models.mock.ts

@@ -0,0 +1,38 @@
+import { type LanguageModel } from 'ai';
+
+const createMockModel = (): LanguageModel => {
+  return {
+    specificationVersion: 'v2',
+    provider: 'mock',
+    modelId: 'mock-model',
+    defaultObjectGenerationMode: 'tool',
+    supportedUrls: [],
+    supportsImageUrls: false,
+    supportsStructuredOutputs: false,
+    doGenerate: async () => ({
+      rawCall: { rawPrompt: null, rawSettings: {} },
+      finishReason: 'stop',
+      usage: { inputTokens: 10, outputTokens: 20, totalTokens: 30 },
+      content: [{ type: 'text', text: 'Hello, world!' }],
+      warnings: [],
+    }),
+    doStream: async () => ({
+      stream: new ReadableStream({
+        start(controller) {
+          controller.enqueue({
+            type: 'text-delta',
+            id: 'mock-id',
+            delta: 'Mock response',
+          });
+          controller.close();
+        },
+      }),
+      rawCall: { rawPrompt: null, rawSettings: {} },
+    }),
+  } as unknown as LanguageModel;
+};
+
+export const chatModel = createMockModel();
+export const reasoningModel = createMockModel();
+export const titleModel = createMockModel();
+export const artifactModel = createMockModel();