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.

[github-actions]

Results for build job (at 585c479):

(No pr-comment-body.txt artifact found from this job.)

[shaoboyan091]

BTW, this http://127.0.0.1:8080/standalone/?q=webgpu:web_platform,external_texture,video:importExternalTexture,video_frame_display_size_scale:videoName=%22four-colors-h264-bt601.mp4%22;displayScale=%22same%22 fail in canary.

[haoxli]

BTW, this http://127.0.0.1:8080/standalone/?q=webgpu:web_platform,external_texture,video:importExternalTexture,video_frame_display_size_scale:videoName=%22four-colors-h264-bt601.mp4%22;displayScale=%22same%22 fail in canary.

I tested it pass on Intel GPU, but failed on NV GPU. It seems to be a tolerance issue or decoder differences on NVIDIA. I will check it again after we update the video file.