Add vehicle listing support to admin client

The Admin Network has no native packet for listing individual vehicles,
so list_vehicles() sends a "list_vehicles" command over the existing
GameScript JSON channel and relies on a companion server-side script to
reply with vehicle data via ServerGamescript. Requires subscribing to
Gamescript updates (documented in docs/PROTOCOL.md) to receive the reply.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

main_admin.py

@@ -48,7 +48,7 @@ async def run_admin():
         await admin.update_frequency(AdminUpdateType.Gamescript, AdminUpdateFrequency.Automatic)
 
         print("--- Requesting vehicle info via GameScript ---")
-        await admin.send_gamescript({"command": "list_vehicles"})
+        await admin.list_vehicles()
         
         await asyncio.sleep(5)
         print("--- Quitting ---")