added simple test for struct

tests/device_struct_test.py

@@ -0,0 +1,24 @@
+"""Unittest for device builder."""
+
+import unittest
+
+from custom_components.bluetti_bt.bluetti_bt_lib.utils.commands import ReadHoldingRegisters
+from custom_components.bluetti_bt.bluetti_bt_lib.utils.struct import DeviceStruct
+
+class TestDeviceStruct(unittest.TestCase):
+    def test_get_read_holding_registers(self):
+        struct = DeviceStruct()
+
+        struct.add_swap_string_field("device_type", 110, 6)
+        struct.add_sn_field("serial_number", 116)
+        struct.add_decimal_field(
+            "power_generation", 154, 1
+        )
+        struct.add_uint_field("total_battery_percent", 102)
+
+        registers = struct.get_read_holding_registers()
+
+        self.assertGreater(len(registers), 0)
+
+        for reg in registers:
+            self.assertIsInstance(reg, ReadHoldingRegisters)