ObserveCountChanged misses count if observer has side effect

If an observer modifies the underlying collection in ObserveCountChanged, the new count will not be published.

    [Fact]
    public void ObserveCountChanged_WithSideEffect()
    {
        var events = new List<int>();
        var collection = new ObservableList<int>([]);

        using var _ = collection.ObserveCountChanged().Subscribe(count =>
        {
            events.Add(count);
            // Side effect - when count is 1, clear the list
            if(count == 1) collection.Clear();
        });

        events.Should().BeEmpty();

        collection.Add(12);

        // collection: { }  (empty)
        // events: { 1 }    (expected:  { 1, 0 }

        collection.Count.Should().Be(0);   
        events.Should().BeEquivalentTo([1, 0]); // <- fails here  
    }

Expected events to be a collection with 2 item(s), but {1} contains 1 item(s) less than {1, 0}

Start by locating the ObserveCountChanged implementation and the existing test named ObserveCountChanged_WithSideEffect. Run that test to reproduce the missing notification when the observer clears the collection; done means the test passes with events containing [1, 0] and the collection count remaining 0.