[WebGPU] Add GridSample operator (#28264)
### Description
Implements the GridSample operator (opset 16–19) for the WebGPU EP.
### Motivation and Context
GridSample was missing from the WebGPU EP and all other major execution
providers already support it. The GridSample tests were extended to
cover the WebGPU EP and seem to pass successfully.
I haven't tested that the `onnxruntime-web` build would pick up this new
operator implementation because it's really hard to build this locally,
but it seems like it should just work.
Closes #27085
