test(update): mock gateway discovery now that the restart phase is surfaced

The gateway auto-restart phase used to swallow every exception at debug
level, so tests driving cmd_update end-to-end never noticed it touching
real gateway discovery. With #78574 surfacing an aborted restart as a
failed update, an unmocked find_gateway_pids on a box with a live
gateway hits the conftest live-system guard and turns into a spurious
sys.exit(1).

Add an autouse fixture in test_cmd_update.py (discovery returns nothing,
systemd unsupported) and the same seams in test_update_head_moved_gate's
helper so the phase is a clean no-op for tests that do not assert on
gateway restarts.

tests/hermes_cli/test_cmd_update.py

@@ -71,6 +71,24 @@ def _patch_managed_uv(request):
         yield
 
 
+@pytest.fixture(autouse=True)
+def _patch_gateway_discovery():
+    """Keep cmd_update's gateway auto-restart phase off this machine's gateways.
+
+    The restart phase used to swallow every exception at debug level, so these
+    end-to-end tests never noticed it touching real gateway discovery. Since
+    the phase is surfaced (#78574: an aborted restart now fails the update),
+    an unmocked ``find_gateway_pids`` on a box with a live gateway reaches the
+    conftest live-system guard and turns into a spurious ``sys.exit(1)``.
+    Discovery returning nothing makes the phase a clean no-op for every test
+    in this module (none of them assert on gateway restarts).
+    """
+    with patch("hermes_cli.gateway.find_gateway_pids", return_value=[]), \
+         patch("hermes_cli.gateway.supports_systemd_services", return_value=False), \
+         patch("hermes_cli.gateway.find_profile_gateway_processes", return_value=[]):
+        yield
+
+
 class TestCmdUpdateNpmLockfileCache:
     @staticmethod
     def _cache_file(hermes_root, project_root):

tests/hermes_cli/test_update_head_moved_gate.py

@@ -105,6 +105,20 @@ def _patch_update_deps(monkeypatch, tmp_path, run_side_effect):
     monkeypatch.setattr(hermes_main, "_clear_update_incomplete_marker", lambda: None)
     # Gateway restart path (called after a successful update).
     monkeypatch.setattr(hermes_main, "_finish_dashboard_update_cleanup", lambda *a: None)
+    # Keep the (now surfaced — #78574) gateway auto-restart phase away from
+    # this machine's real gateways: discovery returns nothing, systemd is
+    # unsupported, so the phase is a clean no-op for both snapshots.
+    import hermes_cli.gateway as hermes_gateway
+
+    monkeypatch.setattr(
+        hermes_gateway, "find_gateway_pids", lambda all_profiles=False: []
+    )
+    monkeypatch.setattr(
+        hermes_gateway, "supports_systemd_services", lambda: False
+    )
+    monkeypatch.setattr(
+        hermes_gateway, "find_profile_gateway_processes", lambda *a, **k: []
+    )
 
 
 def test_update_success_when_head_moves(monkeypatch, tmp_path, capsys):