The podman_gateway_start e2e test shells out to `podman` directly (unlike
every other Podman e2e test, which routes through the gateway). The harness
`e2e/with-podman-gateway.sh` clobbers `XDG_CONFIG_HOME` with an empty dir to
isolate CLI/SDK gateway metadata. On macOS the podman client resolves its VM
connection config from `XDG_CONFIG_HOME`, so a bare `podman ps` falls back to a
nonexistent native rootless socket and fails with "unable to connect to Podman
socket". On Linux the socket resolves via `XDG_RUNTIME_DIR`, so the test passes
there and this is a macOS-only false failure.
Target the same API socket the gateway uses by passing `--url unix://$SOCKET`
when the harness-exported `OPENSHELL_PODMAN_SOCKET` is set, mirroring the
shell's `podman_cmd` helper. When the var is unset (running the test outside the
harness), fall back to plain `podman`, so Linux behavior is unchanged.
Signed-off-by: Russell Bryant <rbryant@redhat.com>