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.

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

@@ -1491,30 +1491,13 @@ public sealed class ModbusDriver
     ///     Map a Modbus logical type to the driver-agnostic <see cref="DriverDataType"/> used
     ///     by the address-space builder.
     /// </summary>
-    /// <remarks>
-    ///     <para>
-    ///         <b>Driver.Modbus-007 — Int64 / UInt64 surfacing limitation:</b>
-    ///         <see cref="DriverDataType"/> does not yet include an Int64 enum member, so 64-bit
-    ///         Modbus tags currently surface as <see cref="DriverDataType.Int32"/> on the OPC UA
-    ///         address space. The wire codec (<c>DecodeRegister</c> / <c>EncodeRegister</c>) is
-    ///         correct — values round-trip as 64-bit <c>long</c> / <c>ulong</c> through
-    ///         <c>ReadAsync</c> / <c>WriteAsync</c>. Only the variable node's <c>DataType</c>
-    ///         attribute is misreported. Clients that consume the type advertisement will see a
-    ///         type/value mismatch for values outside the 32-bit signed range. Operators
-    ///         configuring <c>I_64</c> / <c>UI_64</c> tags should be aware of this until the
-    ///         tracked <c>DriverDataType.Int64</c> follow-up ships.
-    ///     </para>
-    /// </remarks>
-    private static DriverDataType MapDataType(ModbusDataType t) => t switch
+    internal static DriverDataType MapDataType(ModbusDataType t) => t switch
     {
         ModbusDataType.Bool or ModbusDataType.BitInRegister => DriverDataType.Boolean,
         ModbusDataType.Int16 or ModbusDataType.Int32 => DriverDataType.Int32,
         ModbusDataType.UInt16 or ModbusDataType.UInt32 => DriverDataType.Int32,
-        // Driver.Modbus-007: Int64 / UInt64 currently surface as Int32 because DriverDataType
-        // has no Int64 member yet. The wire codec preserves the 64-bit value; only the OPC UA
-        // node's declared DataType is widened. Tracked for a follow-up that adds the enum
-        // member + node-type advertisement.
-        ModbusDataType.Int64 or ModbusDataType.UInt64 => DriverDataType.Int32,
+        ModbusDataType.Int64 => DriverDataType.Int64,
+        ModbusDataType.UInt64 => DriverDataType.UInt64,
         ModbusDataType.Float32 => DriverDataType.Float32,
         ModbusDataType.Float64 => DriverDataType.Float64,
         ModbusDataType.String => DriverDataType.String,