wgsl: Add more validation tests for length

Lifting the pattern from other builtin validation tests, this adds more different kinds of invalid arguments, and covers the @must_use property.

Issue: #

Requirements for PR author:

[X] All missing test coverage is tracked with "TODO" or .unimplemented().
[X] New helpers are /** documented */ and new helper files are found in helper_index.txt.
[X] Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
[X] Test have be tested with compatibility mode validation enabled and behave as expected. (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.

gpuweb / cts