Background Support: Allow modern color functions in standalone gradients

[aaronrobertshaw]

What?

Brings the changes from the Core backport (WordPress/wordpress-develop#11384) back to Gutenberg, and moves the gradient KSES filter into the WordPress 7.1 compat folder.

Follow up to #75859.

Why?

The Core backport updates safecss_filter_attr() directly to allow gradient background values. This PR aligns Gutenberg with tweaks made to Core's approach and relocates the shim so it is removed automatically once these changes reach Gutenberg's minimum supported WordPress version.

How?

• Moves the filter from lib/experimental/kses.php into lib/compat/wordpress-7.1/kses.php, so it is deleted along with that folder once 7.1 is the minimum supported version.
• Updates the logic to match Core. It removes each gradient from the test string and re-checks the remainder, which allows gradients using functions beyond rgb() and rgba() (such as hsl(), oklch() and calc()), whether standalone or combined with a url() image.
• Renames the function to gutenberg_allow_extended_gradient_backgrounds to reflect what it now covers.
• Registers the filter unconditionally, like the neighbouring shim. It is a no-op on WordPress 7.1 and later, where Core handles these values natively.
• Extends the background support tests with standalone gradient cases.

Testing Instructions

No manual testing needed. Coverage lives in the background support tests (phpunit/block-supports/background-test.php). Wait for CI and the e2e tests to pass.

[github-actions]

Flaky tests detected in 161758f.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/28548391374
📝 Reported issues:

• [Flaky Test] uploads multiple images via gallery #79789 in /test/e2e/specs/editor/various/client-side-media-processing.spec.js

[github-actions]

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Co-authored-by: aaronrobertshaw <aaronrobertshaw@git.wordpress.org>
Co-authored-by: ramonjd <ramonopoly@git.wordpress.org>

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