docs: add JSDoc for rstack/test

Author: chenjiahan

packages/rstack/src/test.ts

@@ -1 +1,9 @@
+/**
+ * Re-export @rstest/core APIs so users can import test APIs from `rstack/test`.
+ *
+ * @example
+ * ```ts
+ * import { expect, test } from 'rstack/test';
+ * ```
+ */
 export * from '@rstest/core';