Add AbstractFloat bitcast execution tests.

[dj2]

This CL adds tests for the bitcasting of abstract float values. This includes:

• abstract float to f32
• abstract float to vec2
• vec2 to vec4

Stubs are added for the equivalent AbstractInt tests.

Issue: #1609

---

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.