wgsl: Rework how input domain domains are limited

[zoddicus]

Changes from wrap and forward pattern to have an optional utility function that returns the acceptable domains. This helps simplify reasoning about object lifetimes.

Fixes #3201

---

Requirements for PR author:

• All missing test coverage is tracked with "TODO" or .unimplemented().
• New helpers are /** documented */ and new helper files are found in helper_index.txt.
• Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)

Requirements for reviewer sign-off:

• Tests are properly located in the test tree.
• Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Helpers and types promote readability and maintainability.

When landing this PR, be sure to make any necessary issue status updates.

[zoddicus]

Rebased onto my cleanup PRs, to make my life a bit easier. All the new changes are in db5a341

[jiangzhaoming]

Looks good to me, thanks