The injector never offers the RemoteDesktop portal on niri, but inject-probe
ran the portal probe unguarded and reported portal_available: true there.
The probe now reuses the injector's _is_niri_session guard: on niri it skips
the portal and reports portal_skip_reason "niri" with portal_available false.
Off niri the probe runs and reports its answer as before.
Fixes#46