feat(modbus): String + BitInRegister array decode + equipment-tag arrayLength

- DecodeRegisterArray: add String and BitInRegister cases replacing the
  default:throw; each element decoded by reusing DecodeRegister on its
  contiguous register slice → string[] / bool[]
- ModbusEquipmentTagParser.TryParse: read optional arrayLength key from
  TagConfig JSON and thread it into ModbusTagDefinition.ArrayCount
  (null when absent or zero, preserving scalar behaviour)
- ModbusArrayTests: 8 new tests covering the two decode cases and the
  equipment-tag parser/resolver path; 285/285 green

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Contracts/ModbusEquipmentTagParser.cs

@@ -34,9 +34,14 @@ public static class ModbusEquipmentTagParser
             var byteOrder = ReadEnum(root, "byteOrder", ModbusByteOrder.BigEndian);
             var bitIndex = (byte)ReadInt(root, "bitIndex");
             var stringLength = (ushort)ReadInt(root, "stringLength");
+            // isArray / arrayLength — optional keys authored by the typed Modbus tag editor.
+            // When arrayLength > 0 we expose an array tag of that count; otherwise scalar.
+            var arrayLength = ReadInt(root, "arrayLength");
+            int? arrayCount = arrayLength > 0 ? arrayLength : null;
             def = new ModbusTagDefinition(
                 Name: reference, Region: region, Address: (ushort)address, DataType: dataType,
-                Writable: true, ByteOrder: byteOrder, BitIndex: bitIndex, StringLength: stringLength);
+                Writable: true, ByteOrder: byteOrder, BitIndex: bitIndex, StringLength: stringLength,
+                ArrayCount: arrayCount);
             return true;
         }
         catch (JsonException) { return false; }