Native Windows dataset builds fail because virtualenv and command execution assume POSIX paths

Summary

Running a MEDS-DEV dataset integration test from native Windows PowerShell
fails during the demo dataset fixture, before the dataset test itself executes.

There appear to be at least two independent Windows incompatibilities:

1. The generated virtual environment Python is checked without the .exe
   extension.
2. Dataset commands are always written to a Bash script and invoked using a
   native Windows path that Bash cannot interpret.

I'm fine with the idea of native Windows (without WSL) being not supported, in that case we should just mention it somewhere.

Environment

• MEDS-DEV branch: dev
• MEDS-DEV commit: b9c09e7
• Operating system: Windows, native PowerShell
• Python: CPython 3.12.13
• Test dataset: MIMIC-IV demo

Reproduction

uv run pytest -v -s -m integration `
  --test_dataset=MIMIC-IV `
  tests/test_0_datasets.py

Start by running the reproduced PowerShell command for tests/test_0_datasets.py and trace the demo dataset fixture before the dataset test executes. Inspect how the generated virtual environment Python and dataset commands are checked or invoked; done means the integration test reaches the dataset test on native Windows, or native Windows support is explicitly documented as unsupported.