Improve venv resolution for cross-platform test compatibility

[srichs]

Motivation

• Tests on Windows-like environments were failing because virtualenvs can expose the interpreter at either Scripts/python.exe or bin/python, and the code assumed a single layout.
• Subprocess argument differences across platforms caused inconsistent invocation formatting in tests.

Description

• Update venv_python to compute both Scripts/python.exe and bin/python, preferring the OS-native location but falling back to the alternate when present.
• Keep find_venv selection logic unchanged while making interpreter existence checks resilient to both layouts.
• Normalize the interpreter path passed to subprocess.call in run_py by using as_posix() for consistent argument formatting across platforms.

Testing

• Ran pytest -q tests/test_venv.py tests/test_cli.py::test_doctor_reports_detected_venv tests/test_cli.py::test_doctor_reports_missing_venv_with_hint and the targeted tests passed.
• Ran the full suite with pytest -q and all tests passed (91 passed).

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Codex Task