Speed up Windows integration tests by reusing shared venv via copy

[MatthewMckee4]

Summary

• Instead of creating a fresh venv (uv venv + uv pip install) for each of the ~312 integration tests on Windows, create the shared venv once and copy it into each test directory
• This mirrors the Unix approach (which uses symlinks), but uses directory copying since Windows venv scripts have hardcoded paths that break with symlinks
• Removes #[cfg(unix)] gates from SHARED_VENV, get_or_create_shared_venv, and cleanup_old_shared_venvs so they are available on all platforms
• Adds a #[cfg(windows)] copy_dir helper and fixes the venv python path to use Scripts/python.exe on Windows

Test plan

• just test — all 628 tests pass on macOS (Unix path unchanged)
• prek run -a — all pre-commit checks pass
• Windows CI (windows-latest) will exercise the new copy-based path automatically

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 1 untouched benchmark

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_{Comparing fix/windows-shared-venv-copy (6de8b84) with main (15d82f3)}