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>

tests/test_admin.py

@@ -1,10 +1,15 @@
 import pytest
 import asyncio
+import json
 import os
 import monocypher
 from openttd import OpenTTDAdminClient
 from openttd.protocol import PacketAdminType, AdminUpdateType, AdminUpdateFrequency
 
+def decode_gamescript_payload(packet):
+    """Decode the JSON payload of an AdminGamescript packet (2-byte length + 1-byte type + string)."""
+    return json.loads(packet[3:].split(b"\x00")[0])
+
 class MockTransport:
     def __init__(self):
         self._closing = False
@@ -27,6 +32,20 @@ def test_admin_packet_types():
     assert PacketAdminType.ServerWelcome == 104
     assert PacketAdminType.ServerAuthRequest == 128
 
+@pytest.mark.asyncio
+async def test_admin_list_vehicles():
+    client = OpenTTDAdminClient("127.0.0.1", port=3977, admin_name="TestAdmin")
+    proto = MockProtocol()
+    client._protocol = proto
+    client._transport = MockTransport()
+
+    await client.list_vehicles()
+    await client.list_vehicles(company_id=2)
+
+    assert len(proto.sent) == 2
+    assert decode_gamescript_payload(proto.sent[0]) == {"command": "list_vehicles"}
+    assert decode_gamescript_payload(proto.sent[1]) == {"command": "list_vehicles", "company_id": 2}
+
 @pytest.mark.asyncio
 async def test_admin_client_connect_and_actions(monkeypatch):
     client = OpenTTDAdminClient("127.0.0.1", port=3977, admin_name="TestAdmin")