common: replace deprecated wstring_convert with Win32 conversion APIs

[ronaldtse]

Summary

• Replaces the deprecated std::wstring_convert / std::codecvt_utf8_utf16 pair used by the Windows UTF-8 helpers, as reported by @ni4 in Consider replacing std::wstring_convert with appropriate alternative for Windows. #2302.
• The helpers now call MultiByteToWideChar and WideCharToMultiByte, which are the supported conversion APIs on modern Windows toolchains and avoid the known Microsoft STL memory leak in wstring_convert.
• The throwing-on-error contract of the previous helpers is preserved, and the existing Windows Unicode CLI tests continue to exercise the conversion path.

Test plan

• the windows-native and windows-msys2 matrices pass
• the existing Unicode CLI tests continue to pass on Windows

[ronaldtse]

@ni4 per your issue #2302, this replaces the deprecated std::wstring_convert with the Win32 conversion APIs, preserving the throwing contract and keeping the existing Windows Unicode tests as coverage. Whenever convenient, a review would be appreciated.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.45%. Comparing base (26482f6) to head (fa1f3e5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2493   +/-   ##
=======================================
  Coverage   85.45%   85.45%           
=======================================
  Files         125      125           
  Lines       23042    23042           
=======================================
  Hits        19691    19691           
  Misses       3351     3351           

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