Fix naga immediate_slots_required and panic when var<immediate> size overflows 256 bytes

[beicause]

Connections
Depends on #9711 and #9597

Immedaites issue: #8556

Description

1. Fixes Naga immediate_slots_required calculation is incorrect #9724

2. Fixes panic when var<immediate> is larger than 256 bytes / the bitmask overflows. And validates immediate size of pipeline auto layout.

Testing

For 1, test with CTS webgpu:api,validation,encoding,programmable,pipeline_immediate:required_slots_set:*
For 2, test with CTS webgpu:api,validation,pipeline,immediates:*.

Squash or Rebase?

Rebase

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.