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>

lib/openttd/client.py

@@ -322,6 +322,13 @@ class OpenTTDAdminClient:
         from .protocol import AdminUpdateType
         await self.poll(AdminUpdateType.CompanyStats, company_id)
 
+    async def list_vehicles(self, company_id=None):
+        """Request a list of vehicles via GameScript. company_id=None for all companies."""
+        payload = {"command": "list_vehicles"}
+        if company_id is not None:
+            payload["company_id"] = company_id
+        await self.send_gamescript(payload)
+
     async def send_gamescript(self, json_data):
         """Send a JSON string to the GameScript."""
         import json