VideoPress: improve escaping of block attributes rendered into inline styles

[vianasw]

Proposed changes

The VideoPress video block renders two author-supplied block attributes into inline CSS, and neither was escaped ideally for its context:

• Poster URL — rendered into a background-image: url( … ). esc_url() does not escape every character that is significant inside a CSS url(), so the value is now emitted as a double-quoted CSS string (url("…")) to keep it contained.
• maxWidth — rendered into max-width: …. It now only accepts a plain CSS length or percentage; any other value is dropped and the block falls back to full width (as with the 100% default). Note calc() is also dropped, which the dimension control does not produce.

Both are escaping/validation hardening for values that go into inline styles. Adds render regression tests.

Does this pull request change what data or activity we track or use?

No.

Testing instructions

• Add a VideoPress block with a custom poster (preview-on-hover) and a max width; on the front end the poster and sizing still render correctly.
• Confirm unusual poster or max-width values are handled safely — the poster stays inside url("…") and a non-length max-width is ignored rather than applied.
• jp test php packages/videopress — the new regression tests pass.

🤖 Generated with Claude Code

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[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack), and enable the fix/videopress-poster-css-escaping branch.
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bin/jetpack-downloader test jetpack fix/videopress-poster-css-escaping

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[github-actions]

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[jp-launch-control]

Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/videopress/src/class-initializer.php	153/205 (74.63%)	10.13%	-19 💚

Full summary · PHP report · JS report

[vianasw]

Closing this in favor of an internal change so it can be coordinated with related work — nothing further needed here. Thanks!