Support 2D / Columbus View in snapping APIs

### Feature

Client-side and server-side snapping APIs assume a 3D view (`SceneMode.SCENE3d`).

2D or Columbus Views have not been tested.

This issue is filed as a followup to [#13682 review discussion](https://github.com/CesiumGS/cesium/pull/13682#discussion_r3806195189).

What I suggest as a first step:

Test both snapping APIs ([server-side](https://github.com/CesiumGS/cesium/pull/13682) and [client-side](https://github.com/CesiumGS/cesium/pull/13531)) and note actual behavior before building any solution.

### AI acknowledgment

- [ ] I used AI to generate this feature request.
- [ ] (If the above is checked) I have reviewed the AI-generated content before submitting.

Start with the server-side snapping API in PR #13682 and the client-side snapping API in PR #13531. Test both APIs in 2D and Columbus View, compare their behavior with the 3D assumption, and record the observed results before proposing a solution.

Written by the indexing model from the issue text.