Fix ModbusServerContext parameter for pymodbus 3.x

Changed 'slaves' parameter to 'devices' to match pymodbus 3.x API.

Fixes: TypeError: ModbusServerContext.__init__() got an unexpected
keyword argument 'slaves'

Author: claude

emulator/modbus_server.py

@@ -118,7 +118,7 @@ def __init__(self, simulator, port: int = 502, slave_id: int = 1):
 
         # Create server context
         self.server_context = ModbusServerContext(
-            slaves={slave_id: self.device_context},
+            devices={slave_id: self.device_context},
             single=False
         )