Fix BMS not responding to commands

- Use write_gatt_char with response=True (Write With Response) — many BMS
  firmwares ignore Write Without Response packets
- Add 0.5s delay after start_notify before sending first command to avoid
  race where BMS hasn't registered the notification subscription yet

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

custom_components/xiaoxiang_bms/bluetooth_handler.py

@@ -58,6 +58,9 @@ class BmsBluetoothHandler:
         )
         await self._client.connect()
         await self._client.start_notify(RX_CHAR_UUID, self._on_notify)
+        # Give the BMS a moment to register the notification subscription
+        # before we start sending commands — avoids dropped first response
+        await asyncio.sleep(0.5)
         _LOGGER.debug("Connected to BMS at %s", self._address)
 
     async def disconnect(self) -> None:
@@ -126,7 +129,7 @@ class BmsBluetoothHandler:
             self._response_event.clear()
             self._response_data = None
             try:
-                await self._client.write_gatt_char(TX_CHAR_UUID, command, response=False)
+                await self._client.write_gatt_char(TX_CHAR_UUID, command, response=True)
             except BleakError as exc:
                 _LOGGER.error("BLE write failed: %s", exc)
                 return None