wgsl: Add dot4I8Packed and dot4U8Packed execution tests

[Jiawei-Shao]

Issue: #3180 #3247 #3248

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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.

[ben-clayton]

LGTM! Thanks

[jiangzhaoming]

Still looks good to me, thanks!

[dneto0]

LGTM. Thanks!