Fix stale Bluetooth readings and the integration becoming unavailable

[PyFlat-JR]

What does this PR do?

Fixes Bluetooth devices going stale and the integration becoming unavailable with many devices.

Bluetooth on Windows

• Each Bluetooth device ran its own PowerShell per poll, which scanned the whole PnP tree twice. That took 3.3 s per device, and six at once took 13.5 s against the 10 s read timeout, so from about five Bluetooth devices every one went stale ("No signal").
• WindowsBluetoothBatteryReader (replaces PowerShellPnpBatteryReader) walks the Bluetooth PnP nodes once per poll through cfgmgr32 (NativeDeviceProperties): 38 ms for every device. Every Bluetooth source of a poll shares that walk through BluetoothSnapshot, like on macOS.
• Get-PnpDeviceProperty batched over many nodes returned values for only some of them, a different set on every run, so the config-flow picker sometimes showed devices without their level. It uses the same walk now.
• BluetoothPnpLevels keeps the old rules: the level comes from the sibling node sharing the device's address (BLE first), a hand-typed sibling name such as "<device> Hands-Free AG" still resolves, and names compare ignoring case.

Host throttle

• Macro Deck throttles all of a plugin's callbacks together, adb included. With four phones over adb the poll used up that budget, and a refused RegisterWidgetTypeAsync left the whole integration unusable (every widget "unavailable").
• HostCallRetry retries refused calls with backoff; the widget-type registration and the config reads use it.
• adb phones are read two at a time, so the poll never hits the host's four concurrent adb calls.

Checklist

• I have read AGENTS.md and CONTRIBUTING.md.
• dotnet build and dotnet test pass locally (0 warnings, 274 tests).
• I updated README.md / AGENTS.md if this changes what they describe.
• If this touches a device backend: I tested it against the real hardware (see below).

Device/hardware tested against (if applicable)

Windows 11, Bluetooth: soundcore Sleep A30, soundcore Liberty 4 NC, Anker A7908, CORSAIR HS80 MAX, JBL Flip 4, JBL Go 4, POCO F8 Ultra, SC-PMX94. Every level matches the old per-device read, and the picker lists the same devices (HardwareTests.Reads_every_bluetooth_device_in_one_poll). The throttle fix follows a user's log with four adb phones; the throttle itself was not reproduced here.