Test alphaToCoverageEnabled constraints

[Kangz]

That a fragment stage and target 0 must exist and that its format must be blendable and have an alpha channel.

Add componentCount to color formats in format_info.ts as there was no way to know which formats have an alpha channel.

Add an isColorTextureFormatWithAlpha helper.

Fix isTextureFormatBlendable to take texture-formats-tier1 into account.

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 c41cf0a):

+webgpu:api,validation,render_pipeline,multisample_state:alpha_to_coverage,fragment_stage_required:* - 4 cases, 8 subcases (~2/case)
+webgpu:api,validation,render_pipeline,multisample_state:alpha_to_coverage,first_target_required:* - 4 cases, 8 subcases (~2/case)
+webgpu:api,validation,render_pipeline,multisample_state:alpha_to_coverage,first_format_blendable_and_has_alpha:* - 4 cases, 156 subcases (~39/case)
-TOTAL: 285540 cases, 2349827 subcases
+TOTAL: 285552 cases, 2349999 subcases