Fix struct_padding required size to include trailing padding

[shaoboyan091]

Per community discussion (gpuweb/gpuweb#5563), the group decided that pipeline immediateSize validation counts trailing padding bytes in the struct layout. The struct Data { a: A, @align(32) b: B } has its last member end at byte 40, but the full struct size is 64 due to alignment rounding. Update kRequiredSize from 40 to 64 to match this stricter validation rule.

Issue: #

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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.)
• 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.
• Test descriptions are accurate and complete.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Tests avoid over-parameterization (see case count report).

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

[github-actions]

Results for build job (at abc3273):

 Test case/subcase counts did not change.

[shaoboyan091]

Trailing bytes related small fix for cts, PTAL, thanks!

[Kangz]

Actually this shouldn't be needed no? Only the pipeline layout validation requires 64, but not the immediates set on the encoder.

[shaoboyan091]

Ah, good point. The layout size required 64 but setImmediates required 40. I'll make an update.