fix(plugin-nested-docs): preserve published version when resaving children with drafts

[wingding12]

Fixes #14943

What

Fixes child documents with drafts losing their published version when a parent document is saved.

Why

When a parent document is published, the resaveChildren hook updates all child documents to refresh breadcrumbs. When a child has both a published version and a newer draft version, both were updated sequentially. The second update (draft) caused createVersion to set latest = false on the just-created published version, making it inaccessible on the live site — the child page effectively disappeared.

How

Updated the resaveChildren hook to skip draft versions for children that also have a published version. Only the published version receives the breadcrumb update. Draft-only children (those that have never been published) continue to be updated normally.

This avoids the latest flag corruption without requiring changes to the core versioning logic in createVersion.

Testing

Added two integration tests to test/plugin-nested-docs/int.spec.ts:

1. Published version preserved — creates a parent + published child, adds a draft edit to the child, re-publishes the parent, then verifies both the published and draft versions of the child remain accessible with correct breadcrumbs.
2. Draft-only children still updated — creates a parent + draft-only child (never published), updates the parent, then verifies the draft child's breadcrumbs are refreshed.