fix(rdp): prepend /run/wrappers/bin to PATH and add pkexec preflight check

GRD 50.x invokes pkexec internally for every grdctl --system call, even
when the caller is root.  An isolated systemd script PATH does not include
/run/wrappers/bin automatically, causing exit 70 at boot.

Changes:
- Prepend /run/wrappers/bin to PATH at script start so every subsequent
  grdctl --system resolves the NixOS setuid pkexec wrapper.
- Add an explicit preflight check (test -x /run/wrappers/bin/pkexec) with
  a clear error message before the first grdctl_system call.
- pkgs.polkit remains absent from the service path.
- Update test_setup_runs_grdctl_directly_as_root to reflect that pkexec
  now appears in the preflight check (not as a direct invocation).
- Add test_run_wrappers_bin_prepended_to_path and test_pkexec_preflight_check.

modules/rdp.nix

@@ -51,6 +51,13 @@ lib.mkIf config.sovran_systemsOS.features.rdp {
     script = ''
       set -euo pipefail
 
+      # GRD 50.x invokes pkexec internally for every grdctl --system call, even
+      # when the caller is root.  NixOS exposes the required setuid wrapper at
+      # /run/wrappers/bin/pkexec; the Nix-store polkit binary is not setuid and
+      # must not shadow it.  Prepend the wrapper directory so every subsequent
+      # grdctl --system resolves the correct binary.
+      export PATH="/run/wrappers/bin:$PATH"
+
       STATE_DIR="/var/lib/gnome-remote-desktop"
       TLS_DIR="$STATE_DIR/tls"
       USERNAME_FILE="$STATE_DIR/rdp-username"
@@ -166,6 +173,15 @@ lib.mkIf config.sovran_systemsOS.features.rdp {
       chown gnome-remote-desktop:gnome-remote-desktop "$CRED_FILE"
       chmod 600 "$CRED_FILE"
 
+      # Preflight: the NixOS setuid pkexec wrapper must be present and executable
+      # before any grdctl --system call.  Absence means the system was booted
+      # without security.wrappers or the wrapper directory is not mounted yet.
+      if ! test -x /run/wrappers/bin/pkexec; then
+        echo "Preflight check failed: /run/wrappers/bin/pkexec is absent or not executable." >&2
+        echo "GNOME Remote Desktop system configuration requires the NixOS setuid pkexec wrapper at /run/wrappers/bin/pkexec." >&2
+        exit 1
+      fi
+
       grdctl_system rdp enable
       grdctl_system rdp set-tls-cert "$TLS_DIR/rdp-tls.crt"
       grdctl_system rdp set-tls-key "$TLS_DIR/rdp-tls.key"