Fix missing publishedAt timestamp in Updater service

[Copilot]

The Updater service was not setting publishedAt when changing post status to published, creating an inconsistency with Creator which sets it automatically.

Changes

• src/Cms/Services/Post/Updater.php: Added business rule to auto-set publishedAt when status becomes STATUS_PUBLISHED (only if not already set)
• tests/Cms/Services/Post/UpdaterTest.php: Added test coverage for the new behavior and preservation of existing timestamps

// Business rule: auto-set published date when changing to published status
if( $status === Post::STATUS_PUBLISHED && !$post->getPublishedAt() )
{
    $post->setPublishedAt( new \DateTimeImmutable() );
}

This prevents posts with STATUS_PUBLISHED but null publishedAt, matching the pattern established in Creator.

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