[BUG] App crashes when closed during GenAIModel initialization

[AndrewKeepCoding]

Describe the bug
It takes a few seconds to instantiate Model. The app crashes if you close the app.

GenAIModel.cs

private Task InitializeAsync(string modelDir, CancellationToken cancellationToken = default)
{
    return Task.Run(
        () =>
        {
            _model = new Model(modelDir);
            _tokenizer = new Tokenizer(_model);
        },
        cancellationToken);
}

To Reproduce
Steps to reproduce the behavior:

1. Run the app on the debugger and go to Generate Text.
2. Download a model if you don't have any, restart the app then go to Generate Text again.
3. Close the app while the progress ring is in progress.
4. See error

Expected behavior
The app should close without any errors.

Screenshots
If applicable, add screenshots to help explain your problem.

Please complete the following information:

• App Version [e.g. 0.4.1 from the Microsoft Store]:

Additional context
This might be the same for other models.

[nmetulev]

We likely need to capture the close event and wait for the sample to clean up resources. @azchohfi ideas?

[azchohfi]

Yes, that is pretty much it. Working on a PR.