[webgpu] Fix the wrong fallback in Attention (#26608)
Attention input handling updates:
* Corrected the input indices for `past` from `input[5]` to `input[4]`
in the fallback logic, ensuring the code reflects the actual input
order.
With this change, the Attention ops in phi-4-mm-vision.onnx can go to
the gpu instead of cpu.
