add qubes_pure_sanitize_string_safe_for_display

[piotrbartman]

Added function replaces unacceptable characters with _. I considered replacing _ with the REPLACEMENT CHARACTER (U+FFFD) but the advantage of _ is readability and the single-byte size.

The internal validate_utf8_char function has been renamed to validate_utf8_char_safe_for_display, and the internal logic has been moved to validate_utf8_char_and_return_len, which returns the minus length of unsafe characters. This allows more efficient replacement of multi-byte UTF chars.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.96%. Comparing base (40f1963) to head (0ce2e5c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #136      +/-   ##
==========================================
+ Coverage   60.60%   61.96%   +1.35%     
==========================================
  Files           6        6              
  Lines         952      986      +34     
  Branches      104      104              
==========================================
+ Hits          577      611      +34     
  Misses        285      285              
  Partials       90       90              

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