Switch to connect→poll→disconnect per cycle (single-connection BMS fix)

The BMS only allows one simultaneous BLE connection. Keeping a persistent
connection blocked the mobile app from connecting at all.

Changes:
- BmsBluetoothHandler: replace connect()/disconnect()/request() public API
  with a single poll() method that owns the full connect→read→disconnect
  lifecycle. Connection is held only for the duration of one data fetch.
- Coordinator: gutted connection-state management — _async_update_data now
  just calls handler.poll() and processes results. No reconnect loop needed.
- Poll interval defaults: 15s default, 10s min, 300s max. The BLE connect
  overhead (~2s) makes sub-10s intervals impractical.

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

custom_components/xiaoxiang_bms/const.py

@@ -5,9 +5,9 @@ DOMAIN = "xiaoxiang_bms"
 CONF_ADDRESS = "address"
 CONF_POLL_INTERVAL = "poll_interval"
 
-DEFAULT_POLL_INTERVAL = 5   # seconds — fast for solar/energy monitoring
-MIN_POLL_INTERVAL = 2
-MAX_POLL_INTERVAL = 60
+DEFAULT_POLL_INTERVAL = 15  # seconds — each poll does a full BLE connect/disconnect
+MIN_POLL_INTERVAL = 10      # below this the BMS has no breathing room between polls
+MAX_POLL_INTERVAL = 300
 
 # GATT UUIDs (Xiaoxiang BMS UART-over-GATT)
 UART_SERVICE_UUID = "0000ff00-0000-1000-8000-00805f9b34fb"