src: restrict MaybeStackBuffer string helpers to text types

[anonrig]

Limit MaybeStackBuffer::ToString() and ToStringView to textual element types so byte buffers do not instantiate std::basic_string[_view] on libc++.

This avoids the macOS/Xcode deprecation warning for char_traits<unsigned char> while preserving existing string helper behavior for text buffers. uint16_t buffers are mapped to char16_t for these conversions so UTF-16 call sites continue to work.

[jasnell]

This should just be a concept

   template <typename T>
  concept MaybeStackBufferStringLike =
      std::is_same_v<T, char> || std::is_same_v<T, wchar_t> ||
      std::is_same_v<T, char8_t> || std::is_same_v<T, char16_t> ||
      std::is_same_v<T, char32_t> || std::is_same_v<T, uint16_t>;

// ...

template <MaybeStackBufferStringLike U = T,
        typename Char = MaybeStackBufferStringType<U>>

[targos]

Issue: #62506

There's a PR from a first time contributor at #62507

[anonrig]

Issue: #62506

There's a PR from a first time contributor at #62507

I'll close. Let's get a new contributor!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.70%. Comparing base (a9ac9b1) to head (69be72f).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #62564   +/-   ##
=======================================
  Coverage   89.70%   89.70%           
=======================================
  Files         695      695           
  Lines      214213   214214    +1     
  Branches    41021    41024    +3     
=======================================
+ Hits       192167   192169    +2     
+ Misses      14113    14109    -4     
- Partials     7933     7936    +3     

Files with missing lines	Coverage Δ
src/util.h	90.98% <100.00%> (+0.07%)	⬆️

... and 26 files with indirect coverage changes

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