Fix: AsObservable immediately calls Dispose on completion

[TORISOUP]

Resubmitting #331.

As pointed out in #330, when using AsObservable() in combination with message-delaying operators such as Delay or ObserveOn, the OnCompleted signal was not properly propagated downstream.

var subject = new Subject<Unit>();
var fakeFrameProvider = new FakeFrameProvider();

subject
    .AsObservable()
    .ObserveOn(fakeFrameProvider)
    .Subscribe(
        onNext: _ => Console.WriteLine("Next"),
        onCompleted: _ => Console.WriteLine("Completed")
    );

subject.OnNext(Unit.Default);
fakeFrameProvider.Advance();
subject.OnCompleted();
fakeFrameProvider.Advance();

// "Completed" is not printed

The cause was that WrappedObserver is defined with AutoDisposeOnCompleted = true, and AsObservable() uses it as-is.
As a fix, AsObservable() now uses a dedicated observer.

A similar issue was also present in AsSystemObservable(), so it has been fixed as well.

[Copilot]

Pull request overview

Fixes a completion-propagation bug when AsObservable() (and AsSystemObservable()) is used in operator chains that delay notifications, where OnCompleted could be lost due to premature disposal.

Changes:

• Update AsObservable() to subscribe with a dedicated observer that does not auto-dispose on completion.
• Update AsSystemObservable() bridge observer behavior (and add a new regression test for delayed completion on AsObservable()).
• Add a unit test covering AsObservable() composed with DelayFrame(...) to ensure completion is delivered.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/R3.Tests/OperatorTests/AsObservableTest.cs	Adds a regression test ensuring delayed completion is observed after AsObservable() + DelayFrame.
src/R3/Operators/AsObservable.cs	Changes AsObservable() to avoid premature disposal on completion; adjusts AsSystemObservable() observer behavior.

Suppressed comments (1)

tests/R3.Tests/OperatorTests/AsObservableTest.cs:46

• PR description says a similar completion-propagation issue was fixed in AsSystemObservable(), but the tests here only cover the basic (non-delayed) AsSystemObservable path. Please add a regression test that composes AsSystemObservable() with a message-delaying System.Reactive operator (e.g., Delay(TimeSpan.Zero) or ObserveOn(...)) and asserts OnCompleted is eventually observed downstream.

    [Fact]
    public void AsSystemObservable()
    {
        {
            var p = new Subject<int>();
            var l = new List<int>();
            Exception? ex = null;
            bool completed = false;
            p.AsSystemObservable().Subscribe(l.Add, e => ex = e, () => completed = true);

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[TORISOUP]

Closing this to explore a different approach.