Add dual source blending blend factors to `kBlendFactors`

[Jiawei-Shao]

This patch adds all the blend factors defined in the extension "dual-source-blending" to kBlendFactors in capability_info.ts so that all the tests about blend factors can test these newly added blend factors.

This patch also removes the validation tests about using the alpha channel of Src1 because it doesn't follow the latest SPEC: only when the color blend factor uses the alpha channel of Src must the fragment output be vec4. When the color target doesn't have the alpha channel, the alpha blend factor is ignored so it is OK to set it as if it "uses" the alpha channel of Src.

Note that the new validation test against Src1-alpha will be added into 'pipeline_output_targets,blend' after the fix is landed in the browser.

Issue: #3810

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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 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.