Allow for waiting for DialogHost animations.

[Keboo]

Potential Fix for #4017

Introduces WaitForOpened and WaitForClosed methods on DialogHost to enable asynchronous waiting for the completion of the dialog's visual state transitions. This provides more precise control for executing logic precisely after the dialog has fully opened or closed, rather than just when events are dispatched.

Outstanding items:

1. Are the WaitFor* methods named properly?
2. Tests, need to have them. Will be impossible to test race conditions reliably, but happy path should be there.
3. Should the CancelationToken int he WaitFor* methods be required rather than optional?

Updates the DialogSession to expose the parent DialogHost instance, facilitating access to these new awaitable methods.

[nicolaihenriksen]

Only some minor comments. Looks good to me though.

[corvinsz]

Looks great already, good idea.

1. Are the WaitFor* methods named properly?

LGTM

2. Tests, need to have them. Will be impossible to test race conditions reliably, but happy path should be there.

Agree

3. Should the CancelationToken int he WaitFor* methods be required rather than optional?

Besides what @nicolaihenriksen already mentioned (using a default timeout if none is supplied), what about adding an overload that takes a TimeSpan?

public Task WaitForClosed(TimeSpan timeout)
{
    using CancellationTokenSource cts = new(timeout);
    return WaitForClosed(cts.Token);
}

4. Shouldn't the WaitFor* methods be static, or am I misunderstanding the design of the public API?
   Edit:
   I overlooked your example and now understand how you intended the API: register for DialogClosingEventHandler and call await eventArgs.Session.DialogHost.WaitForClosed(); in the handler.
   To improve discoverability (and maybe developer experience?), thoughts on adding static methods for WaitFor* like so?

public static Task WaitForOpened(object? dialogIdentifier = null, CancellationToken cancellationToken = default)
{
    var instance = GetInstance(dialogIdentifier);
    return instance.WaitForOpened(cancellationToken);
}

public static Task WaitForClosed(object? dialogIdentifier = null, CancellationToken cancellationToken = default)
{
    var instance = GetInstance(dialogIdentifier);
    return instance.WaitForClosed(cancellationToken);
}

// usage:
await DialogHost.Show(...);
await DialogHost.WaitForClosed();