Sanitize attribute names in fenced_code and legacy_attrs

[naaz234]

Description

fenced_code (the non-Pygments path, with attr_list enabled) and legacy_attrs write a user-supplied attribute name into the output without sanitizing it. In fenced_code the key of a key=value pair in a fence's {...} is emitted as f' {k}="..."' with only the value escaped. In legacy_attrs the name from {@name=value} is set straight onto the element with el.set(match.group(1), ...) and the serializer writes attribute names verbatim. The attr_list scanner allows a name of [^ =}]+ and legacy_attrs allows [^}]*, both of which permit >, <, / and quotes, so a name like x><svg/onload=alert(1) closes the <code> tag (or the surrounding element) and injects a live element.

attr_list already guards the element path with sanitize_name (NAME_RE.sub('_', name)), which is where a name like this belongs. This moves that helper to module scope and applies it at the two sites that skipped it, so a name is reduced to a valid XML name before it reaches the output. Valid names (letters, digits, -, ., _, :) are unchanged, so existing attribute lists render the same.

Reproduction before the change:

>>> import markdown
>>> markdown.markdown('``` { .lang x><svg/onload=alert(1) }\ncode\n```', extensions=['fenced_code', 'attr_list'])
'<pre><code class="language-lang" x><svg/onload="alert(1)">code\n</code></pre>'
>>> markdown.markdown('text {@x><svg/onload=alert(1)=y} more', extensions=['legacy_attrs'])
'<p x><svg/onload=alert(1)="y">text  more</p>'

In both cases the > in the name closes the enclosing tag and a live <svg onload=...> element follows. After the change the name becomes x_svg_onload, so it stays inside the intended tag.

Added regression tests for both extensions and a changelog entry.

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Checklist

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[waylan]

Good catch. As this change only adds sanitation in the 2 places it was getting missed, this is an acceptable fix. Thanks for the work on this.