Validate pipeline layout immediate_size and fix its calculation when there are multiple immediate variables

[beicause]

Connections
#8556

Description

Current size_for_module is incorrect, which doesn't consider multiple immediate variables. We should check the immediate size used by entry points.

wgpu/naga/src/valid/immediates.rs

Line 58 in d21c0d0

pub fn size_for_module(module: &crate::Module) -> u32 {

Also the validation is missing. Per spec https://gpuweb.github.io/gpuweb/#abstract-opdef-validating-gpuprogrammablestage

8. If there is an immediate data variable that is statically used by entryPoint (assert there is at most one):

Let T be the store type of the immediate data variable.

Let requiredSize be SizeOf(T).

layout.[[immediateSize]] must be ≥ requiredSize.

Testing
I updated naga snapshots and added a new test in wgpu-validation/immediates.rs, which fails on trunk branch but it is fixed in this PR: cargo nextest run --test wgpu-validation render_multi_immediates_with_all_immediates_set_succeeds

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.