Pandas follow-ups: categorical conversion + threaded starter guardrails

[t-kalinowski]

Summary

• fix direct py_to_r() conversion for pandas.Categorical on pandas 3 by adding short-class S3 dispatch (pandas.Categorical) and switching categorical value extraction to to_numpy()
• reject free-threaded CPython builds in virtualenv_starter() and can_be_virtualenv_starter() using a python -VV probe for free-threaded build markers
• keep the intentional py_require warning assertion in its dedicated test, and disable that warning path in test-python-virtual-environments.R so it does not leak uncaptured output from subprocess stderr

Testing

• devtools::test()
• devtools::test(filter = 'python-envs')
• devtools::test(filter = 'python-virtual-environments')
• devtools::test(filter = 'py_require')
• interactive check in fresh session: py_require('pandas<3') with pandas 2.3.3 and py_to_r(pd$Categorical(...)) returns ordered factor