Enable pytest-xdist runs for py-polars tests (#18016)

This PR reduces CI time by parallelizing pytests execution for `py-polars` tests.

This PR:
```
========== 14748 passed, 28 skipped, 140 xfailed in 112.37s (0:01:52) ==========
```

On `branch-25.04`:
```
== 14748 passed, 28 skipped, 58 deselected, 140 xfailed in 529.08s (0:08:49) ===
```

Authors:
  - GALI PREM SAGAR (https://github.com/galipremsagar)

Approvers:
  - Matthew Roeschke (https://github.com/mroeschke)
  - Gil Forsyth (https://github.com/gforsyth)

URL: #18016

ci/run_cudf_polars_polars_tests.sh

@@ -48,7 +48,9 @@ python -m pytest \
        --cache-clear \
        -m "" \
        -p cudf_polars.testing.plugin \
-       -v \
+       -n 8 \
+       --dist=worksteal \
+       -vv \
        --tb=native \
        $DESELECTED_TESTS_STR \
        "$@" \