Add shader validation tests on subgroup-size-control - Part I

[Jiawei-Shao]

This patch adds the first part of the shader validation tests on the extension subgroup-size-control:

• Enabling subgroup_size_control must also enable subgroups.
• The WGSL attribute @subgroup_size must be used when subgroup_size_control is enabled.
• @subgroup_size can only be used in the compute stage

Issue: #4640

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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 1756791):

+webgpu:shader,validation,extension,subgroup_size_control:enable_subgroup_size_control_requires_subgroups:* - 2 cases, 2 subcases (~1/case)
+webgpu:shader,validation,extension,subgroup_size_control:use_subgroup_size_attribute_requires_subgroup_size_control_extension_enabled:* - 2 cases, 2 subcases (~1/case)
+webgpu:shader,validation,extension,subgroup_size_control:subgroup_size_attribute_only_valid_in_compute_stage:* - 3 cases, 3 subcases (~1/case)
-TOTAL: 281247 cases, 2322858 subcases
+TOTAL: 281254 cases, 2322865 subcases