fix: return futures from virtual device and network session APIs

[mortenboye]

Fixes #173.

Problem

MethodChannelMidiCommand dropped the pigeon call with unawaited, so a platform failure escaped as an unhandled asynchronous error:

PlatformException(AUDIOERROR, Error -2 while create MIDI virtual source, null, null)
  at MidiHostApi.addVirtualDevice (midi_api.g.dart:331)

Wrapping the call in try/catch could not help, because the future was discarded inside the platform implementation rather than returned to the caller.

Fix

addVirtualDevice, removeVirtualDevice and setNetworkSessionEnabled now return Future<void> end to end — MidiCommand → MidiCommandPlatform → MethodChannelMidiCommand → host API:

try {
  await midi.addVirtualDevice(name: 'My App');
} on PlatformException catch (err) {
  // The virtual source was not created.
}

setNetworkSessionEnabled had the identical unawaited defect one line below the two named in the issue; including it here keeps this to a single breaking-signature change rather than two.

Updated in the same shape: the platform-interface base class, and the web, windows and linux implementations. Web now throws asynchronously via async so its "unsupported" error arrives through the future rather than synchronously.

Compatibility

• Existing calls that ignore the result keep compiling (midi.addVirtualDevice(name: 'x'); is still a valid statement).
• Breaking for custom MidiCommandPlatform implementations: the three overrides must widen from void to Future<void>. Documented as migration item 7 in the README.

Verification

• New test in test/flutter_midi_command_test.dart asserting a platform error reaches a MidiCommand caller, plus a success-path test.
• New test in the platform interface asserting PlatformException propagates out of MethodChannelMidiCommand.
• The example app now awaits addVirtualDevice and reverts its virtual transport toggle when creation fails — previously it silently kept the toggle on.
• flutter analyze clean; root, platform-interface, linux, windows, web (chrome) and example suites pass.