wgsl: Implement AbstractFloat addition execution tests

[zoddicus]

There is no compound tests, since += is not defined for consts, and AbstractFloats are always const.

Lots of little tweaks in the PR wrt to renaming things to distinguish between AbstractFloat and AbstractInt, and threading generics through to allow reusing the same code for number and bigint.

Issue #3390

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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.)

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.