Image CDN abilities: register status reads via Registrar

[enejb]

Summary

Adds a minimal Abilities API surface to the jetpack-image-cdn package via the shared Registrar base class. Exposes a single read-only ability — jetpack-image-cdn/get-status — that returns the current Image CDN (Photon) activation state, effective CDN domain, srcset wiring flag, and the deduplicated list of supported MIME types.

Per RSM "Abilities Everywhere" plan §4.2, this package is consumed by both Jetpack's Photon module and Boost, so wiring lives in the package's actions.php (canonical guarded pattern from projects/packages/publicize/actions.php). Registration is gated behind the jetpack_wp_abilities_enabled filter (default false) inside Registrar::init() — no behavior change until the filter is opted in.

get-statistics from the original plan was dropped because the package tracks no local statistics — the audit found no bytes-saved, request-count, or last-request timestamp surface. update-settings and clear-cache writes are intentionally deferred; this PR is reads-only.

Surface

• jetpack-image-cdn/get-status — { active, settings: { quality, formats, srcset_enabled, cdn_domain }, supported_mime_types: [string] }. quality and formats are null because the consumer plugin (not this package) owns those toggles.

Permissions

current_user_can( 'manage_options' ) — the package has no domain-specific capability of its own; reuse the site-admin cap.

Test plan

• composer phpunit -- --filter Image_CDN_Abilities_Test (21 tests, 62 assertions, all green)
• Set add_filter( 'jetpack_wp_abilities_enabled', '__return_true' ) on a site running this branch with the Image CDN module active; confirm wp_get_abilities() includes jetpack-image-cdn/get-status and that /wp-json/wp-abilities/v1/abilities exposes it.
• Confirm the rollout filter default-off path: with the filter omitted, the ability does not appear in wp_get_abilities().
• Override jetpack_photon_domain with a custom host; confirm the returned settings.cdn_domain reflects it.
• CI: PHPCS, parallel-lint, and the full package suite remain green (one pre-existing test failure in Image_CDN_Core_Test::test_photon_url_filter_url_encodes_path_parts is unrelated to this change).

[github-actions]

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• 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 or WordPress.com Site Helper), and enable the add/ship-image-cdn-abilities branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack add/ship-image-cdn-abilities

bin/jetpack-downloader test jetpack-mu-wpcom-plugin add/ship-image-cdn-abilities

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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/image-cdn/src/class-image-cdn.php	379/510 (74.31%)	0.20%	-1 💚

1 file is newly checked for coverage.

File	Coverage
projects/packages/image-cdn/src/abilities/class-image-cdn-abilities.php	71/71 (100.00%) 💚

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