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/ModbusRtuFraming.cs

@@ -105,8 +105,9 @@ public static class ModbusRtuFraming
         }
         else
         {
-            // Write-echo response (FC 05/06/15/16) or any other length-less fixed shape:
-            // fixed 4 payload bytes + CRC(2).
+            // Fixed 4-byte echo: correct for the write FCs this driver emits (05/06/0F/10). A future
+            // variable-length response FC outside 01-04 (e.g. FC23) would be mis-sized here and
+            // surface as a desync — revisit the FC-shape table if the driver starts emitting one.
             trailing = 4 + 2;
         }
 

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."),
         };
     }
 }

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

@@ -19,4 +19,9 @@ public sealed class ModbusTransportFactoryTests
     public void Default_options_build_tcp_transport()
         => ModbusTransportFactory.Create(new ModbusDriverOptions())
             .ShouldBeOfType<ModbusTcpTransport>();
+
+    [Fact]
+    public void Unknown_transport_mode_throws()
+        => Should.Throw<ArgumentOutOfRangeException>(
+            () => ModbusTransportFactory.Create(new ModbusDriverOptions { Transport = (ModbusTransportMode)999 }));
 }