fix: cancel ALSA receive subscription on disconnect for clean shutdown

[thorinside]

Problem

On Linux, disconnecting a MIDI device (or shutting down the app) can hang indefinitely. The root cause is that the StreamSubscription from _device.receivedMessages.listen() is never cancelled before the ALSA device is closed, leaving an active read loop blocking the isolate.

Additionally, _setupStreamController and _rxStreamController are not closed during teardown, and iterating _connectedDevices while modifying it during teardown causes a ConcurrentModificationError.

Fix

• Store the StreamSubscription returned by receivedMessages.listen()
• Cancel the subscription in disconnect() before closing the ALSA device
• Close both _setupStreamController and _rxStreamController during teardown
• Use .toList() when iterating _connectedDevices during teardown to avoid concurrent modification

Related

The underlying dart_midi package also has an ALSA shutdown hang that is fixed separately: maks/dart_midi#8. That PR closes the ALSA port before killing the receive isolate, which complements this fix at the flutter_midi_command_linux level.

Testing

Verified on Linux with:

• Disconnect during active MIDI streaming — returns immediately, no hang
• App shutdown with connected device — exits cleanly
• Repeated connect/disconnect cycles — no crashes or resource leaks

Developed and tested as part of nt_helper.

[maks]

Thanks for this fix @thorinside 👍🏻
FYI @mortenboye I have now merged @thorinside PR with the fix on the dart_midi package side and published it as 0.1.1