🧪 Add regression test for code fences inside GFM alert blocks

[Copilot]

Code fences weren't closing properly when nested inside GFM alert quote blocks (> [!NOTE]), causing content to leak outside the alert. The fix is already present in markdown-it-py 4.2.0 (the minimum version required by this project). This PR adds a regression test to prevent future breakage.

• Added [alert-code-fence] fixture covering multiple fenced code blocks inside an alert

> [!NOTE]
> Note
>
> ```py
> "code"
> ```
>
> other
>
> ```py
> "other code"
> ```