Code theme selector is not appearing when markdown contains code

For whatever reason, it looks like the `containsCode` function appears to not be working everywhere. There's an open task on production where it looks like there is definitely code in the markdown but the code theme selector does not display.

先找出 containsCode 函式和程式碼主題選擇器的邏輯，然後使用明顯包含程式碼的 Markdown 重現回報的案例。將偵測結果與選擇器的顯示條件進行比較。當包含程式碼的 Markdown 能一致地使選擇器顯示，並且為失敗案例加入回歸測試時，即表示完成。

由索引模型根據 Issue 內容生成。