windows: open files via std::filesystem::path so libc++ accepts wide paths

[jcelerier]

base/file_sys.h's path::open() constructs a std::ifstream directly from the std::wstring w_path_ on Windows:

#ifdef _WIN32
    return std::ifstream(w_path_, mode);

That basic_ifstream overload taking a wide string is a Microsoft-STL extension. libc++ (the clang Windows toolchain) does not provide it, so the header fails to compile:

error: no matching constructor for initialization of 'std::ifstream'
   54 |     return std::ifstream(w_path_, mode);
   note: no known conversion from 'const std::wstring' to 'const char *'

This fixes it by constructing a std::filesystem::path from the wide string and using basic_ifstream's filesystem::path constructor, which is standard C++17 and honoured by libc++, libstdc++ and MSVC STL alike — while still preserving the UTF-16 path for Unicode filenames.

Fixes #1071 (and the earlier #985).

[Copilot]

Pull request overview

This PR fixes Windows builds using clang/libc++ by avoiding the non-standard MSVC-only std::ifstream(std::wstring, ...) overload, instead opening files via the standard C++17 std::ifstream(std::filesystem::path, ...) path-based constructor to preserve UTF-16 filenames.

Changes:

• Add <filesystem> to support standard path-based stream constructors.
• Update ort_extensions::path::open() on Windows to construct an std::ifstream from std::filesystem::path(w_path_).

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