Improve custom backend capabilities

[inner-daemons]

Connections
Split off from #9570

Description
Lets custom backends do more things. In particular, this adds as_custom to several API types (which custom backends require to actually make use of these types), and makes render bundles usable on custom backends.

Testing
Tested via gfx-rs/wgpu-native#594

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.

[Copilot]

Pull request overview

This PR expands wgpu custom backend support by exposing additional custom backend accessors and enabling custom render bundle encoder finishing.

Changes:

• Adds new custom-backend accessors for TLAS, BLAS references, and external textures.
• Adds custom request-device error construction/display support.
• Reworks render bundle encoder custom dispatch with a boxed object-safe finish_boxed path.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
wgpu/src/dispatch.rs	Adds object-safe finish_boxed to render bundle encoder interface.
wgpu/src/backend/wgpu_core.rs	Implements finish_boxed for core render bundle encoder.
wgpu/src/backend/webgpu.rs	Implements finish_boxed for WebGPU render bundle encoder.
wgpu/src/backend/custom.rs	Replaces custom render bundle encoder wrapper with boxed trait-object wrapper.
wgpu/src/api/tlas.rs	Adds TLAS custom accessor and lowest_unmodified accessor.
wgpu/src/api/render_bundle_encoder.rs	Routes custom render bundle encoder finish through finish_boxed.
wgpu/src/api/external_texture.rs	Adds external texture custom accessor.
wgpu/src/api/device.rs	Adds custom request-device error variant and constructor.
wgpu/src/api/blas.rs	Adds accessor for a TLAS instance’s custom BLAS.
CHANGELOG.md	Adds changelog entry for custom backend improvements.

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