Add sparse color attachment cases to sample_mask alphaToCoverage test

[shrekshao]

Issue: #2201

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Win D3D12 and Linux Vulkan, the alphaToCoverage mask still uses shader output to target[0].alpha value. (test passes)

On Mac Metal, it looks like when target[0] is empty, it always uses alpha = 1 for the alphaToCoverage mask

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.