🧪 Add regression test for code fences inside GFM alert blocks (#1142)

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

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

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: chrisjsewell <2997570+chrisjsewell@users.noreply.github.com>
Co-authored-by: Chris Sewell <chrisj_sewell@hotmail.com>

Author: Copilot

tests/test_renderers/fixtures/docutil_syntax_extensions.txt

@@ -255,6 +255,35 @@ content
             Second paragraph.
 .
 
+[alert-code-fence] --myst-enable-extensions=alert
+.
+> [!NOTE]
+> Note
+>
+> ```py
+> "code"
+> ```
+>
+> other
+>
+> ```py
+> "other code"
+> ```
+.
+<document source="<string>">
+    <note>
+        <paragraph>
+            Note
+        <literal_block classes="code py" xml:space="preserve">
+            <inline classes="s2">
+                "code"
+        <paragraph>
+            other
+        <literal_block classes="code py" xml:space="preserve">
+            <inline classes="s2">
+                "other code"
+.
+
 [gfm_autolink] --myst-enable-extensions=gfm_autolink
 .
 Visit www.example.com for info.