Show "Immediately" for custom status posts in block editor

[GaryJones]

Summary

Closes #925.

When a post sits in an Edit Flow custom status (Pitch, Assigned, In Progress, and so on), the block editor's Publish field displays a concrete date like "Today at 2:44 pm" rather than "Immediately". Without Edit Flow active, the same underlying post correctly shows "Immediately" — so this is a regression introduced by the REST API round-trip, not the database layer.

The root cause lives in WP core. WP_REST_Posts_Controller::prepare_item_for_response() only returns null for date_gmt when a post's status is draft or pending. Any other status — including every Edit Flow custom status — falls through to the branch that converts 0000-00-00 00:00:00 into a concrete ISO 8601 date derived from post_date. The block editor reads that value, interprets it literally, and renders the confusing concrete date.

The existing fix_custom_status_timestamp() already preserves 0000-00-00 00:00:00 in the database on save, and update_post_date_on_publish_from_custom_status() handles the publish transition correctly, so posts do publish at the right time. The bug is purely cosmetic — but it still undermines editorial trust in what the Publish field is telling them.

The fix adds a rest_prepare_{post_type} filter, registered for every post type that supports custom statuses, which restores parity with core's draft/pending handling. When the post is in a custom status and post_date_gmt is unset, the response's date_gmt is nulled so Gutenberg's PostScheduleLabel falls back to "Immediately". Posts with an explicitly scheduled GMT date are left alone, and draft/pending posts continue to flow through core's own logic untouched.

A new integration test file (CustomStatusRestApiDateTest) covers the happy path plus the obvious regression traps: published posts keeping their concrete date, draft posts still getting null from core, scheduled pitches preserving their explicit GMT date, and pending posts remaining unaffected.

Test plan

• CI: integration tests and PHP lint pass on the branch
• Manual: on a fresh WP install with Edit Flow active, create a post and save it with the default "Pitch" status; confirm the block editor sidebar shows "Immediately" rather than a concrete date
• Manual: schedule a post in a custom status with an explicit future GMT date; confirm that scheduled date is still shown
• Manual: confirm published posts continue to show their publish date in the sidebar
• Manual: deactivate Edit Flow and reload the same post; behaviour matches core (unchanged)