fix(test): add platform guard for grp import

Tests in test_gateway_service.py imported grp inline without a
platform guard, causing ImportError on systems where grp is
unavailable (e.g. macOS, WSL without grp module).

Added pytest.importorskip('grp') at module level alongside the
existing pwd guard, and removed three redundant inline import grp
statements.

Fixes #24531

tests/hermes_cli/test_gateway_service.py

@@ -8,6 +8,7 @@ from types import SimpleNamespace
 import pytest
 
 pwd = pytest.importorskip("pwd")
+grp = pytest.importorskip("grp")
 
 import hermes_cli.gateway as gateway_cli
 from gateway import status
@@ -1331,7 +1332,6 @@ class TestSystemServiceIdentityRootHandling:
 
     def test_explicit_root_is_allowed(self, monkeypatch):
         """When root is explicitly passed via --run-as-user root, allow it."""
-        import grp
 
         root_info = pwd.getpwnam("root")
         root_group = grp.getgrgid(root_info.pw_gid).gr_name