The previous pre-flight check called ctrl.start() without ctrl.attach(),
causing MobileScannerController to time out after 500 ms with
controllerNotAttached on every device — the scanner never worked even
when the plugin was present.
Replace with a direct invokeMethod('state') probe on the scanner channel.
MissingPluginException (thrown when the plugin is not registered) is the
exact error from the crash report; all other errors are surfaced by the
MobileScanner widget's own error builder.
Also add widget tests for the AccountReceiveScreen step-2 import flow
(text fallback, successful import, invalid-QR error) which were
previously untested.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
