[WebGPU] Fix SkipSimplifiedLayerNormalization bias (#28427)
### Description
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The op spec for com.microsoft.SkipSimplifiedLayerNormalization puts the
optional 1-D bias at input slot 3 (the simplified variant has no beta),
but onnxruntime/contrib_ops/webgpu/bert/skip_layer_norm.cc hardcodes
beta = Input(3); bias = Input(4) regardless of the simplified template
parameter — so the user-supplied bias is silently consumed as beta and
the kernel reads slot 4 (out of range for this op) for the actual bias.
The CPU EP handles the slot correctly; only WebGPU diverges (and
produces NaN in larger graphs where slot 4's memory is non-zero).
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Closes https://github.com/microsoft/onnxruntime/issues/28424
