Null out host handle in OleInterfaces when disposing

[JeremyKuhne]

This was causing the AxHost's container to stay rooted.

Fixes #13499

Microsoft Reviewers: Open in CodeFlow

[JeremyKuhne]

The repro of the customer scenario in ScratchProject looks like this:

internal static class Program
{
    private const string WebBrowserClsidString = "8856f961-340a-11d0-a96b-00c04fd705a2";

    [STAThread]
    public static void Main()
    {
        Application.EnableVisualStyles();
        Application.SetHighDpiMode(HighDpiMode.PerMonitorV2);

        for (int i = 0; i < 10; i++)
        {
            Load(loadAxHost: true);
            GC.Collect();
            GC.WaitForPendingFinalizers();
        }

        GC.Collect();
        GC.WaitForPendingFinalizers();
    }

    private static void Load(bool loadAxHost)
    {
        var form = new Form1();
        if (loadAxHost)
        {
            SubAxHost ax = new(WebBrowserClsidString);
            form.Controls.Add(ax);
        }

        form.Show();
        Application.DoEvents();

        form.Close();
        Application.DoEvents();
    }

    private class SubAxHost : AxHost
    {
        public SubAxHost(string clsid) : base(clsid)
        {
        }
    }
}

Without this change, you can find Form1 in the diagnostic tools memory snapshot at the end of the program. An added finalizer will also run (after the fix) if you reregister for finalization in the Dispose method for Form1. GC.ReRegisterForFinalize(this);

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.59701%. Comparing base (9db17fd) to head (e30aa2b).
Report is 2 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #13532         +/-   ##
===================================================
+ Coverage   76.59556%   76.59701%   +0.00144%     
===================================================
  Files           3230        3230                 
  Lines         639148      639149          +1     
  Branches       47295       47295                 
===================================================
+ Hits          489559      489569         +10     
+ Misses        146010      146003          -7     
+ Partials        3579        3577          -2     

Flag	Coverage Δ
Debug	76.59701% <100.00000%> (+0.00144%)	⬆️
integration	18.79201% <0.00000%> (+0.00343%)	⬆️
production	51.00073% <100.00000%> (+0.00331%)	⬆️
test	97.40411% <ø> (ø)
unit	48.37260% <100.00000%> (-0.02285%)	⬇️

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