fix: update post date to current time when publishing from custom status

[GaryJones]

Summary

Posts published from custom statuses like "Pitch" or "Assigned" were retaining their original creation date rather than updating to the actual publication time. This caused:

• Posts appearing with stale dates in archives/feeds
• SEO issues when dates are in URLs
• Excessive redirects when editors manually correct dates (e.g., with Yoast Pro)

WordPress core updates post_date when publishing from 'draft' or 'pending', but doesn't know about Edit Flow's custom statuses. This fix adds that same behavior for custom statuses.

Changes

• Added update_post_date_on_publish_from_custom_status() method that hooks into wp_insert_post_data
• When transitioning to 'publish' from an unpublished custom status with empty post_date_gmt, the post_date is updated to current time
• Scheduled posts and explicitly set dates are still respected
• Added comprehensive test coverage with 6 test cases

Test plan

• All 6 new tests pass
• PHP linting passes
• Pre-existing test failures are unrelated to this change (verified by testing without the fix)
• Manual test: Create a post with "Pitch" status, wait, then publish - date should update to publish time
• Manual test: Schedule a post for future - date should be preserved when it auto-publishes
• Manual test: Explicitly set a past date when publishing - that date should be respected

Fixes #750

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