Let authors edit checklists in their own comments

[lancewillett]

What changed

Checklist authorization for comments now runs the existing o2_Write_API::restrict_comment_editing() capability filter around the edit_comment check, the same way the comment Edit and Trash actions already do. Authors can tick, edit, delete, and add checklist items in comments they wrote on posts owned by other users. Other users' comments still need moderate_comments.

WordPress core maps edit_comment to the parent post's edit capabilities and never consults the comment's author, so after #255 removed the role-name fallback an Author lost checklist access to their own comments on someone else's post. The rendered checkbox came out disabled and the AJAX request was refused.

The method is now declared public and its parameter renamed to $object_id so the WordPress.com mirror passes its coding-standards gate. Behavior is otherwise unchanged.

Testing

• New PHPUnit coverage in tests/phpunit/modules/test-checklists.php: an Author's own comment on another Author's post is editable and renders enabled; another user's comment is not editable and renders disabled; logged-out users are denied. The first test fails with the fix reverted.
• Full suite: 85 tests, 150 assertions pass locally against WordPress 7.1.
• Browser regression coverage in https://github.com/Automattic/p2/pull/6745: an Author's own comment on another user's post renders an enabled checkbox and accepts an update, while the post owner's comment renders disabled and refuses the update.

[lancewillett]

8ad031c adds a PHPUnit case that an Editor can still edit checklists in another user's comment, covering the moderate_comments branch of the reused filter. Suite: 86 tests, 151 assertions pass.