fix(modbus): surface Int64/UInt64 node DataType (Driver.Modbus-007)

Make MapDataType internal, split the combined Int64/UInt64 arm to return
DriverDataType.Int64 and DriverDataType.UInt64 respectively, and remove
the now-stale Driver.Modbus-007 caveat doc block and inline comment.
Add a Theory covering both cases; full suite 271/271 green.

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

@@ -1,6 +1,7 @@
 using System.Buffers.Binary;
 using Shouldly;
 using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
 using ZB.MOM.WW.OtOpcUa.Driver.Modbus;
 
 namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests;
@@ -278,6 +279,17 @@ public sealed class ModbusDataTypeTests
         ModbusDriver.DecodeRegister(wire, lo).ShouldBe("eH");
     }
 
+    // --- MapDataType node DataType advertisement ---
+
+    /// <summary>
+    /// Verifies that 64-bit Modbus types surface the correct DriverDataType (not the stale Int32 fallback).
+    /// </summary>
+    [Theory]
+    [InlineData(ModbusDataType.Int64, DriverDataType.Int64)]
+    [InlineData(ModbusDataType.UInt64, DriverDataType.UInt64)]
+    public void MapDataType_64bit_surfaces_correct_DriverDataType(ModbusDataType wire, DriverDataType expected)
+        => ModbusDriver.MapDataType(wire).ShouldBe(expected);
+
     // --- BCD (binary-coded decimal, DL205/DL260 default numeric encoding) ---
 
     /// <summary>