Fix WebGPU Conv auto_pad=SAME_UPPER padding calculation (#27249)
The WebGPU Conv and ConvTranspose operators were producing incorrect
results when using auto_pad=SAME_UPPER with strides > 1.
Root cause: The head padding values were being unnecessarily
recalculated after InferPadsAndOutputShape() had already computed the
correct values. The recalculation formula could produce incorrect
results.
Fix: Simply use pads[0] and pads[1] directly, which already contain the
correct head padding values computed upstream. This matches the behavior
of the TypeScript implementation.
Fixes #26734