harden(modbus-rtu): factory throws on unknown transport + document RTU FC-shape assumption

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusTransportFactory.cs

@@ -16,7 +16,11 @@ public static class ModbusTransportFactory
     /// </summary>
     /// <param name="options">Driver configuration options.</param>
     /// <returns>A <see cref="ModbusRtuOverTcpTransport"/> when <see cref="ModbusTransportMode.RtuOverTcp"/>
-    /// is selected; otherwise a <see cref="ModbusTcpTransport"/> (also the default/unrecognized fallback).</returns>
+    /// is selected; a <see cref="ModbusTcpTransport"/> when <see cref="ModbusTransportMode.Tcp"/> is selected.</returns>
+    /// <exception cref="ArgumentOutOfRangeException">
+    /// <paramref name="options"/>.<see cref="ModbusDriverOptions.Transport"/> is not a recognized
+    /// <see cref="ModbusTransportMode"/> member — fail loudly rather than silently falling back to TCP/MBAP.
+    /// </exception>
     public static IModbusTransport Create(ModbusDriverOptions options)
     {
         ArgumentNullException.ThrowIfNull(options);
@@ -27,11 +31,13 @@ public static class ModbusTransportFactory
                 keepAlive: options.KeepAlive,
                 idleDisconnect: options.IdleDisconnectTimeout,
                 reconnect: options.Reconnect),
-            _ => new ModbusTcpTransport(
+            ModbusTransportMode.Tcp => new ModbusTcpTransport(
                 options.Host, options.Port, options.Timeout, options.AutoReconnect,
                 keepAlive: options.KeepAlive,
                 idleDisconnect: options.IdleDisconnectTimeout,
                 reconnect: options.Reconnect),
+            _ => throw new ArgumentOutOfRangeException(
+                nameof(options), options.Transport, "Unknown Modbus transport mode."),
         };
     }
 }