chore(scripts): restore test:validate npm script

Restore the test:validate script that was previously removed. This script runs all validation tests in the tests/validate directory using Vitest with verbose reporting.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ericyangpan

package.json

@@ -20,6 +20,7 @@
     "spell:fix": "cspell '**/*.{ts,tsx,js,jsx,json,md,mdx}' --show-suggestions",
     "test": "vitest",
     "test:ci": "vitest run --reporter=verbose",
+    "test:validate": "vitest run tests/validate --reporter=verbose",
     "test:urls": "RUN_URL_TESTS=1 vitest run tests/validate/urls.accessibility.test.ts --reporter=verbose",
     "validate:i18n": "npx tsx scripts/validate/validate-i18n.ts",
     "validate:i18n-usage": "npx tsx scripts/validate/validate-i18n-usage.ts",