Add CI smoke test for Python/reticulate setup

[Copilot]

This adds a lightweight testthat smoke test to validate the CI Python setup before the broader suite runs. It verifies that Python 3.12, the animl_env virtual environment, animl-py, and the package’s reticulate wiring are all available as expected.

• New CI smoke test

  • Adds tests/testthat/test-python_setup.R
  • Skips cleanly when Python is not available at all
  • Fails when CI has Python but the expected animl environment is misconfigured

• Environment checks

  • Confirms animl_env exists via reticulate::virtualenv_exists("animl_env")
  • Verifies the active interpreter reports Python 3.12

• animl-py import validation

  • Confirms the animl Python module is available to reticulate
  • Imports animl directly and checks that the imported module is also present in animl:::.animl_internal$animl_py

• Reticulate integration check

  • Verifies the interpreter seen by reticulate::py_config() matches sys.executable
  • Ensures the package is using the configured Python environment rather than a different interpreter

test_that("CI Python setup is available for animl", {
  expect_true(reticulate::virtualenv_exists("animl_env"))
  expect_equal(
    reticulate::py_eval(
      "'{}.{}'.format(__import__('sys').version_info[0], __import__('sys').version_info[1])"
    ),
    "3.12"
  )
  expect_true(reticulate::py_module_available("animl"))
})