fix(editor): exclude Customizer Additional CSS from iframed editor

[miguelpeixe]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

WordPress 7.0 iframes the post editor whenever every block in the post uses Block API v3+. The iframed editor pulls in the front-end style pipeline, which includes the Customizer's Additional CSS. Any front-end rule that targets `body { ... }` (or other broad selectors) therefore lands on the editor canvas itself, distorting the editing UX on sites that have customized the body element.

This PR filters block_editor_settings_all to drop the specific entry WP core injects for Customizer Additional CSS — identified by __unstableType === 'user' AND isGlobalStyles === false. The front-end is unaffected: the CSS is still printed by wp_custom_css_cb() on wp_head.

A new newspack_strip_customizer_css_in_editor filter is exposed as an escape hatch for sites that intentionally want Customizer CSS in the editor.

Scope notes:

• Only the Customizer entry is removed. The block-theme Global Styles → Additional CSS entry (isGlobalStyles === true) is left untouched.
• Per-block Custom CSS introduced in WP 7.0 is not affected — that's scoped to individual blocks.

How to test the changes in this Pull Request:

1. On a site running WP 7.0, add a rule like body { background: red; } in Appearance → Customize → Additional CSS.
2. Open a post or page that contains only Block API v3+ blocks (the default for current core blocks).
3. Before this fix on release: the editor canvas turns red. After this fix: the editor canvas stays normal; the front-end is still red.
4. Confirm the escape hatch: add add_filter( 'newspack_strip_customizer_css_in_editor', '__return_false' ); in mu-plugins → editor canvas turns red again.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally? (PHPCS clean)

[Copilot]

Pull request overview

This PR prevents Customizer Additional CSS from being applied inside the WP 7.0+ iframed block editor canvas by removing the specific core-injected “user” CSS entry from the editor settings styles array, while leaving front-end output intact. It also introduces a filter to allow sites to opt out of this behavior.

Changes:

• Hooks block_editor_settings_all to remove the Customizer “Additional CSS” style entry from $settings['styles'].
• Adds an escape-hatch filter newspack_strip_customizer_css_in_editor to disable the stripping when desired.

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