Fix WebGPU buffer segment offset alignment

[jchen10]

Align maxStorageBufferBindingSize down to the nearest multiple of minStorageBufferOffsetAlignment after querying device limits. This ensures that when large buffers are split into segments, each segment's byte offset satisfies WebGPU's bind group offset alignment requirement (typically 256 bytes).

[jchen10]

The error to fix:

Exception: WebGPU validation failed. Offset (1073741820) of [Buffer (unlabeled)] does not satisfy the minimum BufferBindingType::ReadOnlyStorage alignment (256).
 - While validating entries[1] against { binding: 1, visibility: ShaderStage::Compute, buffer: {type: BufferBindingType::ReadOnlyStorage, minBindingSize: 2, hasDynamicOffset: 0} }.
 - While validating [BindGroupDescriptor ""Gather""] against [BindGroupLayout (unlabeled)]
 - While calling [Device].CreateBindGroup([BindGroupDescriptor ""Gather""]).

[jchen10]

@fs-eire @guschmue @qjia7 PTAL

[guschmue]

/azp run Linux QNN CI Pipeline,Win_TRT_Minimal_CUDA_Test_CI,Windows ARM64 QNN CI Pipeline,Windows GPU Doc Gen CI Pipeline

[azure-pipelines]

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