Fix XML rendering crash with non-printable characters

[tautschnig]

Handle non-printable characters in XML attributes by escaping them as numeric character references instead of aborting.

Co-authored-by: Kiro autonomous agent

Fixes: #7073

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.01%. Comparing base (76dd20f) to head (18f0b44).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8806   +/-   ##
========================================
  Coverage    80.00%   80.01%           
========================================
  Files         1700     1700           
  Lines       188249   188277   +28     
  Branches        73       73           
========================================
+ Hits        150615   150643   +28     
  Misses       37634    37634           

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[kroening]

My understanding is that this is still invalid XML. So we either error, or produce invalid XML.

[kroening]

To make progress, how about encoding the null-character as \0? This would be better than dropping the character, and better than aborting. The downside is that it is indistinguishable from the sequence "backslash" "zero" unless we wanted to also escape backslashes.