feat(demos): enhance assistant demo with new utility files and mock MCP integration

- Introduced new utility files: `assistantConstants.ts`, `mockMcp.ts`, and `responseProvider.ts` to support enhanced functionality in the assistant demo.
- Updated `Assistant.vue` to utilize the new response provider and mock MCP tool, improving the interaction flow.
- Revised documentation in `assistant.md` to reflect the inclusion of new demo files, ensuring comprehensive examples for users.
- Updated sidebar items in theme configuration for better navigation and clarity on tool usage.

Author: gene9831

docs/.vitepress/themeConfig.ts

@@ -33,9 +33,9 @@ const sharedSidebarItems = [
     text: '工具',
     base: '/tools/',
     items: [
-      { text: 'AI模型交互工具类', link: 'ai-client' },
-      { text: '消息数据管理', link: 'message' },
-      { text: '会话数据管理', link: 'conversation' },
+      { text: 'useMessage 消息数据管理', link: 'message' },
+      { text: 'useConversation 会话数据管理', link: 'conversation' },
+      { text: 'AIClient 模型交互工具类', link: 'ai-client' },
       { text: '工具函数', link: 'utils' },
     ],
   },