feat(wgpu-hal/dx12): add raw_resource method to Buffer

[Fachep]

Description
Allow accessing ID3D12Resource of Buffer.

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[Copilot]

Pull request overview

This PR adds a DX12-specific escape hatch to access the underlying ID3D12Resource for wgpu-hal’s dx12::Buffer, matching existing raw access patterns used elsewhere in the backend.

Changes:

• Add Buffer::raw_resource() to expose the underlying Direct3D12::ID3D12Resource.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Wumpf]

looks like we already have this pattern on Texture, so whatever I don't like about it would also apply here, but that would make it a bigger question and this just makes it consistent 🤷 👍