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_e2e.py

@@ -267,6 +267,40 @@ async def test_e2e_admin_send_gamescript_multiple_inputs(connected_admin):
     await connected_admin.send_gamescript({"command": "ping", "sequence": 1})
     await asyncio.sleep(0.5)
 
+@pytest.mark.e2e
+@pytest.mark.asyncio
+async def test_e2e_admin_list_vehicles_all_companies(connected_admin):
+    # Public function: list_vehicles()
+    # Input 1: all companies (no company_id)
+    responses = []
+    connected_admin.on_gamescript = lambda data: responses.append(data)
+    await connected_admin.update_frequency(AdminUpdateType.Gamescript, AdminUpdateFrequency.Automatic)
+
+    await connected_admin.list_vehicles()
+    await asyncio.sleep(0.5)
+
+    assert not connected_admin.shutdown_event.is_set()
+    assert len(responses) >= 1
+    assert "vehicles" in responses[-1]
+    assert isinstance(responses[-1]["vehicles"], list)
+
+@pytest.mark.e2e
+@pytest.mark.asyncio
+async def test_e2e_admin_list_vehicles_specific_company(connected_admin):
+    # Public function: list_vehicles()
+    # Input 2: specific company_id
+    responses = []
+    connected_admin.on_gamescript = lambda data: responses.append(data)
+    await connected_admin.update_frequency(AdminUpdateType.Gamescript, AdminUpdateFrequency.Automatic)
+
+    await connected_admin.list_vehicles(company_id=0)
+    await asyncio.sleep(0.5)
+
+    assert not connected_admin.shutdown_event.is_set()
+    assert len(responses) >= 1
+    assert "vehicles" in responses[-1]
+    assert isinstance(responses[-1]["vehicles"], list)
+
 
 # --- Protocol Public Functions ---