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/models.py

@@ -274,7 +274,10 @@ class TagMetadata(models.Model):
                 "border": "border-zinc-600/20 dark:border-zinc-400/20",
             },
         }
-        return mapping.get(self.colour, get_auto_tag_colour_css(self.tag.name))
+        css = mapping.get(self.colour)
+        if css is not None:
+            return css
+        return get_auto_tag_colour_css(self.tag.name)
 
 
 class ArticlePageAdminForm(WagtailAdminPageForm):