Add CoRegisterClassObject regression tests for #1670 (no projection change)

[jevansaks]

Investigates #1670. Conclusion: not a CsWin32 bug — the issue is a usage error. This PR adds two regression tests that document the correct vs incorrect call pattern; no source-generator changes.

What the filer did

var thumbnailProviderFactory = new ThumbnailProviderFactory(client);
var thumbnailProviderFactoryInterface = ComWrappers.GetOrCreateComInterfaceForObject(thumbnailProviderFactory, CreateComInterfaceFlags.None);
CoRegisterClassObject(
    typeof(ThumbnailProvider).GUID,
    thumbnailProviderFactoryInterface, // an nint
    (uint)CLSCTX.CLSCTX_LOCAL_SERVER,
    (uint)REGCLS.REGCLS_MULTIPLEUSE,
    out var cookie);

The cswin32-generated parameter is [MarshalAs(UnmanagedType.Interface)] object pUnk. Passing the nint boxes it as System.IntPtr; the LibraryImport-generated ComInterfaceMarshaller<object> then calls ComWrappers again to wrap that boxed IntPtr as a fresh COM object. What gets registered with COM is a wrapper around an IntPtr, not the original IClassFactory. SCM's QueryInterface for IClassFactory then fails, so CoCreateInstance (or out-of-proc activation) never invokes the factory.

What they should do

Pass the managed factory directly — let the source-generated marshaller do the ComWrappers call for you:

PInvoke.CoRegisterClassObject(
    typeof(ThumbnailProvider).GUID,
    thumbnailProviderFactory,    // managed object, NOT a pre-marshalled nint
    CLSCTX.CLSCTX_LOCAL_SERVER,
    REGCLS.REGCLS_MULTIPLEUSE,
    out var cookie);

(ThumbnailProviderFactory must be a [GeneratedComClass] implementing a [GeneratedComInterface] for IClassFactory, which they already had since their hand-written workaround proved their ComWrappers setup was sound.)

If they want explicit control over raw COM pointers (e.g. to use a specific ComWrappers strategy), they can set "allowMarshaling": false in NativeMethods.json, which projects IUnknown* parameters as void* and lets them pass the nint directly via (void*)factoryPunk.

Tests added

Both run on a dedicated STA thread under the existing useComSourceGenerators: true + DisableRuntimeMarshalling=true setup (matches the filer's environment).

• RegisteringManagedFactory_InvokesCreateInstance — happy path. Passes the managed [GeneratedComClass] TestClassFactory directly to PInvoke.CoRegisterClassObject and verifies CoCreateInstance invokes CreateInstance.
• PassingRawPointerAsObject_DoesNotRegisterFactory — documents the broken pattern from the issue. Pre-marshals via ComWrappers, passes the resulting nint, and asserts the factory is never invoked. This locks in the documented behavior so we notice if it ever changes.

Validation

• New tests pass on net9.0 and net10.0
• All other tests in GenerationSandbox.BuildTask.Tests still pass