Studio CI: run the Resolve-CudaToolkit unit test in the Windows GGUF job (#5921)

The deferred Windows CUDA Toolkit check (Resolve-CudaToolkit in setup.ps1) has a
pwsh unit test but nothing ran it. Rather than add a separate workflow, fold a
fast GPU-free gate into the existing Windows GGUF CI: parse setup.ps1 and run
tests/studio/test_resolve_cuda_toolkit.ps1 right after checkout, before the heavy
GGUF smoke, so a setup.ps1 regression fails fast on the runner that is already
spun up.

Refs #5912.