[webgpu] And int64 to cast (#25610)
This pull request extends the WebGPU execution provider to support int64
data type casting in the `Cast` operator, with conditional support based
on whether graph capture is enabled. It refactors kernel registration to
allow toggling int64 support and updates the shader code and kernel
logic to handle int64 tensors efficiently.
It's part of the work to enable graph capture in phi4
https://github.com/microsoft/onnxruntime/pull/25868
