Use scene.pick to get GroundPrimitive and return wrong results

[Sandcastle](https://sandcastle.cesium.com/index.html#c=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)


Start with the linked Sandcastle reproduction and inspect how scene.pick handles the GroundPrimitive shown in the example. Trace the picking result through the scene-rendering path, then verify that picking the primitive returns the expected result rather than the incorrect result shown in the GIF.

Written by the indexing model from the issue text.