Add tvOS support

### What happened?

Opening the the [Unity samples for v1.18.1](https://github.com/CesiumGS/cesium-unity-samples/releases/tag/v1.18.1) with Unity 6 targeting tvOS has the following compile errors:

```
Library/PackageCache/com.cesium.unity@03780efe4504/Runtime/CesiumPointCloudRenderer.cs(101,17): error CS0103: The name 'XRSettings' does not exist in the current context
Library/PackageCache/com.cesium.unity@03780efe4504/Runtime/CesiumPointCloudRenderer.cs(101,51): error CS0103: The name 'XRSettings' does not exist in the current context
Library/PackageCache/com.cesium.unity@03780efe4504/Runtime/CesiumPointCloudRenderer.cs(102,17): error CS0103: The name 'XRSettings' does not exist in the current context
Library/PackageCache/com.cesium.unity@03780efe4504/Runtime/CesiumPointCloudRenderer.cs(102,51): error CS0103: The name 'XRSettings' does not exist in the current context
Library/PackageCache/com.unity.xr.openxr@05fb1a7d75d1/Runtime/input/OpenXRDevice.cs(27,39): error CS1061: 'XRDeviceDescriptor' does not contain a definition for 'characteristics' and no accessible extension method 'characteristics' accepting a first argument of type 'XRDeviceDescriptor' could be found (are you missing a using directive or an assembly reference?)
Library/PackageCache/com.unity.xr.openxr@05fb1a7d75d1/Runtime/input/OpenXRDevice.cs(29,44): error CS1061: 'XRDeviceDescriptor' does not contain a definition for 'characteristics' and no accessible extension method 'characteristics' accepting a first argument of type 'XRDeviceDescriptor' could be found (are you missing a using directive or an assembly reference?)
```
With this compile errors we are unable to compile the project/samples for tvOS

### Environment

Cesium for Unity version: 1.18.1
Unity Editor Version: 6000.0.58f2
Operating System: macOS 26.0.1

### Reproduction steps

1. Download [Unity samples for v1.18.1](https://github.com/CesiumGS/cesium-unity-samples/releases/tag/v1.18.1)
2. Open the project with Unity 6.0 version, for example 6.0.58f2
3. Switch platform to target and compile for tvOS

### Supporting evidence

_No response_

Reproduce the tvOS build using the Unity samples for v1.18.1 with Unity 6.0.58f2. Inspect Runtime/CesiumPointCloudRenderer.cs around the XRSettings errors and the reported com.unity.xr.openxr Runtime/input/OpenXRDevice.cs errors. Done means the project and samples compile successfully when targeting tvOS.

Written by the indexing model from the issue text.