Microsoft.UI.Input.InputPointerSource does not implement IWeakReferenceSource

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### Describe the bug

When subscribing to its events using C++/WinRT auto_revoke, the application crashes during event registration.
```cpp
if (auto xamlRoot = sender.as().XamlRoot())
{
if (auto island = xamlRoot.ContentIsland())
{
auto inputPointerSource = winrt::Microsoft::UI::Input::InputPointerSource::GetForIsland(island);

auto revoker = inputPointerSource.PointerPressed(winrt::auto_revoke, [](auto&, auto&) {});
}
}
```

### Why is this important?

This scenario currently leads to a runtime crash with no obvious indication that the API does not support weak references.

Since most WinRT types support weak references by default, developers can easily run into this issue unintentionally. If this behavior is by design, the documentation should clearly state that `InputPointerSource` does not support weak references.

### Steps to reproduce the bug

The minimal reproduction is shown in the code sample above.

### Actual behavior

_No response_

### Expected behavior

_No response_

### Screenshots

_No response_

### NuGet package version

2.2.0

### Windows version

_No response_

### Additional context

_No response_

Start with the minimal reproduction at InputPointerSource::GetForIsland and the PointerPressed auto_revoke registration shown in the issue. Investigate the InputPointerSource implementation and its weak-reference behavior, then verify that event registration no longer crashes or that the unsupported behavior is clearly documented, with a regression test if the project provides one.

Written by the indexing model from the issue text.