feat(modbus-rtu): add ModbusTransportMode enum (Tcp | RtuOverTcp)

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

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusTransportModeTests.cs

@@ -0,0 +1,15 @@
+using Shouldly;
+using Xunit;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests;
+
+public sealed class ModbusTransportModeTests
+{
+    [Fact]
+    public void Tcp_is_the_default_zero_member()
+        => ((ModbusTransportMode)0).ShouldBe(ModbusTransportMode.Tcp);
+
+    [Fact]
+    public void Exactly_two_members_ship_in_v1()
+        => Enum.GetNames<ModbusTransportMode>().ShouldBe(["Tcp", "RtuOverTcp"]);
+}