md_html: escape raw HTML in image alt attribute

[iliaal]

The HTML renderer opens the alt="..." attribute in render_open_img_span() and renders the image label into it. enter_span_callback() already suppresses nested span tags while image_nesting_level > 0, but text_callback() still emitted MD_TEXT_HTML verbatim, so a raw HTML span in an image label broke out of the attribute:

![<x title=" y" onerror="alert(1)">](/url)

rendered as:

<p><img src="/url" alt="<x title=" y" onerror="alert(1)">"></p>

where onerror becomes a live attribute on the generated img.

This routes MD_TEXT_HTML through render_html_escaped() when inside an image label, matching the renderer's documented intent (only plain text falls into alt), the MD_FLAG_NOHTML output, and cmark-gfm, which escapes raw HTML in the same plain-text context. Behaviour outside image labels is unchanged. Adds a regression test.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.91%. Comparing base (755ce49) to head (a524e1a).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #376      +/-   ##
==========================================
+ Coverage   78.89%   78.91%   +0.02%     
==========================================
  Files           5        5              
  Lines        3790     3794       +4     
  Branches     1270     1271       +1     
==========================================
+ Hits         2990     2994       +4     
  Misses        297      297              
  Partials      503      503              

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