`set_up_kvikio()` silently overrides kvikio thread pool configuration set by callers

`set_up_kvikio()` in libcudf is called lazily on the first IO operation via `std::call_once`. It does two things:

1. One-time initialization: warms the CUDA context (`cudaFree(nullptr)`) and sets compat mode from `KVIKIO_COMPAT_MODE`.
2. Sets the kvikio thread pool size from `KVIKIO_NTHREADS`, defaulting to 4 if unset.

Any caller that sets the thread pool size via the kvikio API before the first IO (say, `kvikio.defaults.set(num_threads=256)`) will have that setting silently overridden when \`set_up_kvikio()\` fires. Note that setting `KVIKIO_NTHREADS` in the environment does work, since `set_up_kvikio()` reads it. But callers should not have to rely on env vars to make their configuration stick.

This is the root cause of the hack in #23683, where cudf-polars has to call `set_up_kvikio()` eagerly (and pass its own thread count) to prevent libcudf from undoing its configuration later.