Harden custom status migration and the publish-timestamp workaround

[GaryJones]

Summary

Two long-standing rough edges in the custom-status module are tightened here.

The web status-migration handler used to reassign every post from one status to whatever target was submitted, without checking that the target was a real status. A typo or a forged value could therefore leave posts stranded in a status that does not exist. The handler now validates the submitted target against the full set of valid statuses — the core statuses plus every registered custom status. That set is exposed as a new public get_all_valid_statuses() method on the module, and the WP-CLI migrate command, which previously carried its own private copy of the same logic, now delegates to it so there is a single source of truth.

Separately, check_timestamp_on_publish is a workaround that writes post_status straight to the posts table when a post with no GMT timestamp transitions to publish, on both the edit.php bulk-edit path and the post.php single-publish path. It did no verification of its own. It now confirms the relevant edit nonce (bulk-posts, or update-post_<id>) and the per-post edit_post capability before writing. Because it runs on admin_init, it bails quietly rather than calling wp_die when a check fails, so legitimate requests are unaffected.

Test plan

• composer cs and parallel-lint pass on the changed files.
• Two integration tests were added and pass: one confirms the valid-status allow-list covers both core and custom statuses, the other confirms the publish workaround makes no write without a valid nonce but still applies with one.
• The six pre-existing CustomStatusTest "post name not set" failures are unrelated to this branch and fail identically on develop.

Fixes VIPPLUG-22.