Post Notification Emails: handle Like clicks on WoW sites

[taipeicoder]

Fixes DOTCOM-16569

Proposed changes

• Adds a new post-like-from-email feature in jetpack-mu-wpcom that hooks template_redirect at priority 1.
• Detects email-like-button clicks (?like=1&postid=…&like_actor=…&like_hmac=…), then proxies them to the site-scoped wpcom/v2/sites/<blog_id>/email-like endpoint via Client::wpcom_json_api_request_as_blog() so the HMAC validation and Likes::set_post_like() happen on wpcom infrastructure.
• The JSON body sends post_id (matching the wpcom endpoint's expected field name), like_actor, and like_hmac.
• Always redirects the visitor to the post permalink afterwards (response body intentionally ignored).
• Bails out on IS_WPCOM to avoid double-firing if jetpack-mu-wpcom is loaded inside wpcom.
• Uses get_wpcom_blog_id() from utils.php to resolve the blog ID, matching the pattern used by replace-site-visibility.php.
• Wires the new feature into Jetpack_Mu_Wpcom::load_features() (alphabetical, unconditional — the email recipient is anonymous, so the handler must run for visitors who are not logged into wpcom).

Related product discussion/links

• Companion wpcom PR (endpoint): Automattic/wpcom#211090
• Email template render PR (previously reverted, can be re-landed once this ships): Automattic/wpcom#210242

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

No. This PR only forwards an HMAC-signed (post_id, like_actor, like_hmac) triple to the wpcom endpoint — no new tracking, telemetry, or PII flow is introduced on the Jetpack side.

Testing instructions

Requires Automattic/wpcom#211090 to be deployed first (the wpcom/v2/sites/<blog_id>/email-like endpoint must exist).

1. Deploy this branch to an Atomic/WoW staging site (ensure wpcom#211090 is also deployed).
2. Add temporary error_log lines around the Client::wpcom_json_api_request_as_blog() call to see the wpcom response.
3. Visit any post URL with ?like=1&postid=<post_id>&like_actor=<wpcom_user_id>&like_hmac=DEADBEEF appended.
4. Expected: browser redirects to the post permalink; debug log shows HTTP 403 invalid_hmac (bad HMAC correctly rejected); no PHP errors.
5. For a full positive-path test, generate a real HMAC via wpl_generate_like_hmac( $post_id, $user_id ) on a wpcom sandbox shell and substitute it into the URL — the like should be recorded.

Negative paths

• Missing query params: handler bails on isset check — no wpcom call, page renders normally
• On wpcom: handler's IS_WPCOM guard bails — the like is already handled on wpcom
• Disconnected site (get_wpcom_blog_id() returns false): handler bails before making any call

Deploy order

1. Automattic/wpcom#211090 (endpoint) — safe standalone, nothing calls it yet
2. This PR (handler) — ships through normal Jetpack release train to Atomic
3. Automattic/wpcom#210242 (email template render) — re-land once both above are deployed

[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 (WordPress.com Site Helper), and enable the fix/dotcom-16569-email-like-atomic branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack-mu-wpcom-plugin fix/dotcom-16569-email-like-atomic

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

Thank you for your PR!

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Follow this PR Review Process:

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[jp-launch-control]

Code Coverage Summary

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Full summary · PHP report

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