Require Actual Attendees before closing an event

[dd32]

Summary

• Shows the Actual Attendees field when event status is wcpt-scheduled (previously only shown after wcpt-closed), so supporters can fill it in before closing
• Adds validation to prevent manual transition to wcpt-closed without filling in the Actual Attendees count
• Cron auto-close (close_wordcamps_after_event) is not affected - it bypasses this validation since it is not an admin form submission and auto-fills the field from CampTix data
• Adds admin error notice when the validation blocks closing

Changes

• wordcamp-admin.php: Changed Actual Attendees visibility condition from wcpt-closed only to wcpt-scheduled or wcpt-closed. Added closed-status validation in require_complete_meta_to_publish_wordcamp(). Added admin notice Require Actual Attendees before closing an event #5.
• New test file test-wordcamp-actual-attendees.php with 5 tests covering: validation blocks close without attendees, allows close with attendees, does not block cron, field visibility at scheduled/pre-scheduled statuses.

Test plan

• Set event to wcpt-scheduled status - verify Actual Attendees field appears
• Try to close event without filling Actual Attendees - verify error notice and status reverts to scheduled
• Fill in Actual Attendees and close - verify it works
• Verify cron auto-close still works (no $_POST context)
• All 5 new PHPUnit tests pass

Closes WordPress#1603

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