fix(tests): make unit tests build on Windows

[raykuo998]

Summary

This PR fixes several Windows compatibility issues in the unit tests that prevented the project from building with llvm-mingw on Windows ARM64.

The production targets (transcribe and transcribe-cli) already built successfully, but the test suite used a few POSIX-specific APIs that are unavailable on Windows.

Changes

• Replace the POSIX-only mkstemps usage with a cross-platform implementation.
• Replace setenv with the appropriate Windows implementation where necessary.
• Fix std::filesystem::path handling so Windows wide-character paths are passed correctly.

Verification

Built successfully on:

• Windows 11 ARM64 (Snapdragon X2 Plus)
• llvm-mingw 20260616 (ucrt-aarch64)
• CMake 4.4.0
• Ninja

Both the library and the complete test suite now build successfully.

[cjpais]

Thanks for this. I'll take a look when I can and try to pull it in. I don't have a Windows ARM machine to test on other than CI, so I'll try to push this through CI and just make sure it works.