Add comprehensive documentation and type safety to test-runner config

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Copilot

.storybook/test-runner.ts

@@ -1,10 +1,21 @@
 import type { TestRunnerConfig } from '@storybook/test-runner';
 
+// Extend Window interface to include __test property
+// Note: This variable is referenced in test-runner's page.evaluate calls
+// but not directly in our codebase. It appears to be expected by the test environment.
+declare global {
+  interface Window {
+    __test?: boolean;
+  }
+}
+
 const config: TestRunnerConfig = {
   async preVisit(page) {
     // Inject __test global to prevent ReferenceError during test execution
+    // This addresses the error: "page.evaluate: ReferenceError: __test is not defined"
+    // that occurs when running Storybook test-runner smoke tests
     await page.evaluate(() => {
-      (window as any).__test = true;
+      window.__test = true;
     });
   },
 };