[wppoland]

Allows password-protected posts to be sticky across all interfaces in WordPress core, following the direction proposed by @wildworks in comment:29 and verified by @jolanc in comment:33 and comment:34:

• REST API (WP_REST_Posts_Controller): Removes the 400 rest_invalid_field rejection when creating or updating a post with both a password and sticky = true.
• Admin (edit_post() and wp_write_post()): Stops unsetting sticky when visibility is password (private posts remain non-sticky).
• XML-RPC (wp_xmlrpc_server::_toggle_sticky()): Relaxes the sticky guard so only private posts are rejected.
• Classic Editor UI (post_submit_meta_box() and src/js/_enqueues/admin/post.js): Keeps the "Stick this post to the front page" checkbox available when "Password protected" is selected and displays Password protected, Sticky when active.
• Tests: Updates and adds PHPUnit coverage across WP_REST_Posts_Controller, edit_post(), and wp_xmlrpc_server::wp_newPost().

Note: PR #13246 implements the narrower alternative (unsticking on Quick Edit when a password is set). Both PRs are available depending on final committer sign-off on Trac.

Trac ticket: https://core.trac.wordpress.org/ticket/64810

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[github-actions]

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[wppoland]

The first CI run had 3 failures, all in WP_Test_REST_Posts_Controller: three update tests still expected the old 400 rest_invalid_field response that this PR removes.

Fixed in 179542e:

• test_update_post_with_password_and_sticky_fails() is removed. It duplicated test_update_post_with_password_and_sticky_succeeds(), added earlier in this PR.
• test_update_stick_post_with_password_fails() is now test_update_stick_post_with_password_succeeds(): a sticky post that gets a password returns 200 and keeps both.
• test_update_password_protected_post_with_sticky_fails() is now test_update_password_protected_post_with_sticky_succeeds(): a password protected post that is made sticky returns 200 and is sticky.

Locally, all 288 WP_Test_REST_Posts_Controller tests pass, as do the edit_post() tests in tests/admin/includesPost.php and the xmlrpc group (sticky and password filters). PHPCS is clean on the changed file.