Fix mobile BLE scan stream completion and cleanup

[robekl]

Summary

Fixes mobile BLE scan stream lifecycle so scan flows complete reliably and do not hang waiting for stream termination.

Problem

MobileBluetoothService.scanForDevices() yielded a controller stream that was never guaranteed to close.
This could leave callers waiting on onDone indefinitely and keep UI scan state stuck.

Changes

• Added _scanController to track the active scan stream controller.
• In scanForDevices():
  • Cancel any previous scan subscription before starting a new one.
  • Guard controller.add(...) with !controller.isClosed.
  • Start a background watcher on FlutterBluePlus.isScanning and close the controller when scanning stops.
  • Ensure controller cleanup in finally.
• In stopScan():
  • Close _scanController if still open.
  • Clear _scanController reference after cleanup.

Why this works

The scan stream now has explicit termination paths for:

• natural scan stop/timeout (isScanning == false)
• manual stop (stopScan())
• error/teardown (finally)

So the consumer can consistently receive stream completion and unblock scan UI logic.