Fix eager evaluation in get_css_classes and auto-slug test

- Replace dict.get() with eager default in TagMetadata.get_css_classes()
  with explicit if/else to avoid unnecessary MD5 hash + DB access
- Fix test_article_save_auto_generates_slug_from_title to actually test
  auto-generation by passing slug="" instead of the expected result

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/blog/tests/test_models.py

@@ -164,13 +164,13 @@ def test_article_save_auto_generates_slug_from_title(home_page):
     author = AuthorFactory()
     article = ArticlePage(
         title="My Great Article",
-        slug="my-great-article",
+        slug="",
         author=author,
         summary="summary",
         body=[("rich_text", "<p>body</p>")],
     )
     index.add_child(instance=article)
-    article.save()
+    article.refresh_from_db()
     assert article.slug == "my-great-article"