[WebGPU] Add the ability to interop with external textures

[AdrianEddy]

Description
Adds two complementary pieces of public API on the WebGPU backend so external interop crates can read and inject GPUTexture handles without copy:

1. as_webgpu() accessors on Texture, TextureView, Buffer, Queue, and Device - return Option<&webgpu::Gpu*>, the WebGPU counterpart of as_hal (which always returns None on this backend, since WebGPU is not a wgpu_hal API).
2. Device::create_texture_from_webgpu_handle(texture, desc) - wraps a foreign webgpu::GpuTexture (e.g. from canvas getCurrentTexture()) as a wgpu::Texture without copy. The wrapped texture is external: its destroy() is a no-op, since the JS handle's lifetime is the caller's responsibility.

The vendored handle types (GpuTexture, GpuTextureView, GpuBuffer, GpuQueue, GpuDevice) are re-exported under a new public module wgpu::webgpu.

The unsafe contract on create_texture_from_webgpu_handle is documented in detail (same-device, descriptor-must-match, lifetime-must-extend); cross-device or descriptor-mismatch surfaces as a WebGPU validation error at first use, which wgpu cannot prevent at construction time because the spec exposes no GPUTexture.device reflection.

Testing
Explain how this change is tested.

Squash or Rebase?

Squash

Checklist

• I self-reviewed and fully understand this PR.
• WebGPU implementations built with wgpu may be affected behaviorally.
• Validation and feature gates are in place to confine behavioral changes.
• Tests demonstrate the validation and altered logic works.
• CHANGELOG.md entries for the user-facing effects of this change are present.
• The PR is minimal, and doesn't make sense to land as multiple PRs.
• Commits are logically scoped and individually reviewable.
• The PR description has enough context to understand the motivation and solution implemented.

[jimblandy]

The description of this sounds really reasonable.

[inner-daemons]

We normally assign PRs to maintainers on Wednesday, but some didn't get assigned this week. I'll try to take a look anyway and it'll most likely be assigned to someone next week.