WI01118904 - Handle disposed AsyncWaitHandle during completion

[jaywang-cn]

Summary

• tolerate ObjectDisposedException when ThreadMethodEntry.Complete signals an AsyncWaitHandle that has already been disposed by its consumer
• add a regression test verifying the callback runs and the async result reaches completion
• use the fork-compatible message loop to process the queued callback

Context

• Work item: WI01118904
• Upstream issue: dotnet/winforms#14996
• Upstream PR: dotnet/winforms#14998 (APPROVED, CI passed)

Testing

• dotnet build .\src\test\unit\System.Windows.Forms\System.Windows.Forms.Tests.csproj
• System.Windows.Forms.Tests.exe --filter-method "System.Windows.Forms.Tests.ControlTests.Control_BeginInvoke_DisposedAsyncWaitHandle_CompletesCallback" --no-progress --output Detailed (1 passed)