Update display size scale tests with video frame from video file

[haoxli]

For display‑size scale tests, a VideoFrame created from a video file is the right source to validate importExternalTexture because it exercises the real decode, color conversion, and display‑size metadata path. A canvas ImageData frame bypasses the video decode path, so it’s removed.

Added a focused video list to validate scaling across codec and color‑space variation, covering both codecs (VP9, H.264) and both color spaces (bt.601, bt.709).

Issue: #4605

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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.