[Metal/hal] Expose raw_device() from Metal Adapter

[AdrianEddy]

Description
This PR adds a raw_device() method on Metal's Adapter. It returns the underlying MTLDevice that adapter wraps.

Mirrors Device::raw_device() for the device side. Useful for callers that need to query Metal-only adapter metadata (e.g. registryID, location, isHeadless) without first requesting a logical device.

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.