[webgpu] use u32 to represent f16 in uniform

[fs-eire]

Description

For f16 uniform variables, use u32 to bit-wise represent them.

Motivation and Context

Some devices supports f16 in shader/storage buffer, but not in uniform buffers. Dawn will set the f16_support to false for them. However, we don't necessarily have to use f16 in uniform.

This change together with #25349 will enable using f16 models on some Android devices.

[Copilot]

Pull Request Overview

This PR updates how 16-bit floats are represented in uniform buffers by packing them into 32-bit unsigned integers and adjusts buffer layout computation and WGSL codegen accordingly.

• Compute size and alignment for f16 uniforms as u32-backed containers in webgpu_context.cc.
• Generate WGSL bitcast expressions in GetElementAt for f16 access in shader_variable.h.
• Change uniform struct declarations to use u32 types and adjusted lengths in shader_helper.cc.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
onnxruntime/core/providers/webgpu/webgpu_context.cc	Revised uniform offset/size logic to handle f16 as u32 with correct alignment and padding.
onnxruntime/core/providers/webgpu/shader_variable.h	Expanded GetElementAt to emit bitcast<vec2<f16>> accesses for f16 uniforms.
onnxruntime/core/providers/webgpu/shader_helper.cc	Mutates data_type/length for f16 and emits appropriate WGSL types (u32, vecN, array).

Comments suppressed due to low confidence (1)

onnxruntime/core/providers/webgpu/webgpu_context.cc:381

• The comment for the f16 array threshold (>8) does not match the implementation (which uses length > 6). Please update the comment to reflect the actual branch condition or adjust the threshold to align with the intended behavior.

    // - length > 8      : array<vec4<u32>, N>   (align 16) (size 16 * N, N = ceil(length / 8))