feat(python/omp-rpc): added user and group options to RpcClient startup

- Added user, group, and extra_groups parameters to RpcClient initialization.
- Propagated those parameters through to subprocess startup calls.
- Added tests to verify the new kwargs are passed correctly, including None defaults and empty extra groups.

python/omp-rpc/tests/test_user_group.py

@@ -0,0 +1,45 @@
+from __future__ import annotations
+
+from unittest.mock import patch
+
+import pytest
+
+from omp_rpc import RpcClient
+
+
+class _Sentinel(Exception):
+    pass
+
+
+def _start_and_capture(**kwargs):
+    client = RpcClient(**kwargs)
+    with patch("omp_rpc.client.subprocess.Popen", side_effect=_Sentinel("aborted")) as mock_popen:
+        with pytest.raises(_Sentinel):
+            client.start()
+    assert mock_popen.call_count == 1
+    return mock_popen.call_args
+
+
+def test_no_user_group_defaults_to_none():
+    call = _start_and_capture(executable="omp")
+    assert call.kwargs["user"] is None
+    assert call.kwargs["group"] is None
+    assert call.kwargs["extra_groups"] is None
+
+
+def test_user_and_group_kwargs_threaded():
+    call = _start_and_capture(
+        executable="omp",
+        user=2001,
+        group="omp",
+        extra_groups=[2000, "docker"],
+    )
+    assert call.kwargs["user"] == 2001
+    assert call.kwargs["group"] == "omp"
+    assert call.kwargs["extra_groups"] == [2000, "docker"]
+
+
+def test_extra_groups_none_distinct_from_empty():
+    call = _start_and_capture(executable="omp", extra_groups=[])
+    # [] means an empty supplementary group list and differs from None.
+    assert call.kwargs["extra_groups"] == []