fix(windows): marshal EventSink calls to platform thread

[OlehHumeniuk]

Description

Fixes threading issue where MediaFoundation callbacks were calling Flutter's EventSink from MTA (Multi-Threaded Apartment) threads instead of the platform thread.

Problem

[ERROR:flutter/shell/common/shell.cc(1120)] The 'xyz.luan/audioplayers/events/...' 
channel sent a message from native to Flutter on a non-platform thread.

[ERROR:flutter/shell/platform/windows/task_runner_window.cc(61)] 
Failed to post message to main thread.

This causes UI freezes and potential crashes during audio loading on Windows.

Root Cause

MediaEngine's IMFMediaEngineNotify::EventNotify() is called on MTA threads, but Flutter requires all Platform Channel messages to be sent from the platform thread.

Solution

• Added PlatformThreadHandler class that creates a hidden message-only window
• Uses Windows message queue (PostMessage/SendMessage) to marshal callbacks to platform thread
• Modified EventStreamHandler to automatically dispatch calls through the handler
• Handler is initialized during plugin registration

Changes

File	Change
platform_thread_handler.h	NEW - Windows message-based thread marshaling
event_stream_handler.h	Modified to use PlatformThreadHandler
audioplayers_windows_plugin.cpp	Initialize/shutdown PlatformThreadHandler
CMakeLists.txt	Added new header file

Testing

• Tested on Windows 10/11
• No threading errors in console
• UI doesn't freeze during audio loading
• Concurrent audio operations work correctly

Notes

• Zero changes to audio_player.cpp - fix is transparent
• Windows-only change, doesn't affect other platforms
• Minimal performance overhead (one PostMessage per event)

[Gustl22]

While we love documentation, adding a comment to each inserted line doesn't really help us and just generates noise. Could you remove the comments where not necessary?

[Gustl22]

Thank you for the contribution. I will need some time to get this reviewed, as I am not a C(++) expert. But I will be happy to move forward with this platform topic.

[Gustl22]

I had a quick look, the approach looks good. I still need to evaluate the thread logic, though.
Ideally this should be done by Flutter, but they still have this issue open:
flutter/flutter#79213
I'm not even sure, if they considered the case of EventChannel: flutter/flutter#150525 (comment)

[Gustl22]

I tried this approach, but didn't get it working within 2 hours. I would prefer that we spent the time in

• Consider adding Windows plugin API to post tasks to main thread flutter/flutter#79213
• or Make Windows plugin APIs thread-safe flutter/flutter#134346

Leave it open as inspiration and reminder to tackle this in flutter for now.