test(gateway): isolate the escaping test from the host's installed unit

Gate review: with the installed-unit fallback, the absent-env assertions read the real
~/.config/systemd/user unit (conftest does not redirect HOME).

tests/hermes_cli/test_gateway_service.py

@@ -332,9 +332,11 @@ class TestGeneratedSystemdUnits:
         assert f"RestartForceExitStatus={GATEWAY_SERVICE_RESTART_EXIT_CODE}" in unit
         assert f"RestartPreventExitStatus={GATEWAY_FATAL_CONFIG_EXIT_CODE}" in unit
 
-    def test_user_unit_carries_ld_library_path_escaped_for_systemd(self, monkeypatch):
+    def test_user_unit_carries_ld_library_path_escaped_for_systemd(self, monkeypatch, tmp_path):
         """#14613: glibc reads LD_LIBRARY_PATH only at process start, so the unit file is the
         only place it can reach CUDA-backed tools; quotes/backslashes must survive systemd quoting."""
+        # The absent-env branch falls back to the installed unit: keep the host's real one out.
+        monkeypatch.setattr(gateway_cli, "get_systemd_unit_path", lambda system=False: tmp_path / "hermes-gateway.service")
         monkeypatch.setenv("LD_LIBRARY_PATH", '/opt/cu"da/lib64:/opt/back\\slash/lib')
 
         unit = gateway_cli.generate_systemd_unit(system=False)