Improve BLE device resolution and WalkingPad GATT locking

[Ceiku]

Summary

• Resolve a connectable BLEDevice via async_ble_device_from_address, with a fallback to async_scanner_devices_by_address (strongest RSSI) when the merged device is missing—common with sparse advertisements or Bluetooth proxies.
• Use a single asyncio.Lock for connect, disconnect, and all GATT commands; the previous _begin_cmd() returned a new lock each time, so operations were not actually serialised.
• Refresh the BLEDevice from Home Assistant before each connect so routing stays current.
• Use device_registry.format_mac for unique_id in the Bluetooth async_step_device path so it matches async_step_bluetooth.

Test plan

• Reload the integration with the WalkingPad powered and in range (local adapter or ESPHome Bluetooth proxy).
• Confirm config entry setup succeeds when the pad is visible only on a secondary scanner path.
• Verify sensors update and belt/speed controls still work when remote control is enabled.

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