fix: extend PHPUnit\Framework\TestCase not WP_UnitTestCase in stub suite

This repo uses stub-based PHPUnit, not WP integration tests.
WP_UnitTestCase is not loaded by bootstrap.php.

Agent-Session: fix-articles-500-20260617

tests/unit/ArticlesListTotalCastTest.php

@@ -5,7 +5,9 @@
  * @package Peptide_News
  */
 
-class ArticlesListTotalCastTest extends WP_UnitTestCase {
+use PHPUnit\Framework\TestCase;
+
+class ArticlesListTotalCastTest extends TestCase {
     /**
      * Verify that $wpdb->get_var returning a numeric string does not
      * cause a TypeError in number_format() (PHP 8.3 strict typing).