test(integration): opt the config pass-through spend-log case into auth (#44265)

The failure spend-log row from #42695 is written for pass-through routes that
run as LLM API routes, which a config route only does with auth: true. The
test omitted auth and passed only while config wins (#41779) registered config
entries through the typed model, where auth defaults to true. #43962 restored
the pre-config-wins registration, so the route lost that status and the row
was never written. Set auth: true on the route so the test covers the logging
it was written for without depending on that side effect

tests/integration/observability/test_passthrough_upstream_error_visibility.py

@@ -238,6 +238,7 @@ def test_config_pass_through_route_logs_body_and_strips_query(gateway: Gateway,
                 "target": f"{wire.url}/upstream?trace=secret-q",
                 "include_subpath": True,
                 "headers": {"Authorization": "Bearer scripted"},
+                "auth": True,
             }
         ]
         path.write_text(yaml.safe_dump(config))