feat(frontend): split test assistant nav into cases and batch pages

Make 测试助手 a sidebar section like 监控观察, with placeholder routes for 测试用例 and 批量测试.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/components/pages/TestCasesPage.tsx

@@ -0,0 +1,10 @@
+import { PlaceholderPage } from "./PlaceholderPage";
+
+export function TestCasesPage() {
+  return (
+    <PlaceholderPage
+      title="测试用例"
+      description="管理助手的测试用例，覆盖典型问答与边界场景。"
+    />
+  );
+}