Fix WebGPU MoE swiglu_limit (default to infinity) (#27221)
### Description
According to
https://github.com/microsoft/onnxruntime/blob/main/docs/ContribOperators.md#commicrosoftmoe,
> swiglu_limit : float
The limit used to clamp in SwiGLU. No clamp when limit is not provided.
However, currently, the default is set to 0, meaning we clamp to 0 if no
limit is provided.
### Motivation and Context
Fixes #27220. See there for bug description and reproduction.
Hoping to get this in before 1.24.0 releases. cc @guschmue
