fix: add retry for appending existing subtitle to iframed editor

[laurelfulford]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

This PR adds a timed retry to attach the subtitle to the post title when populated. Otherwise, in WP 7.0 posts with a populated subtitle don't actually preview them in the editor until they're edited again.

Closes NEWS-1761

How to test the changes in this Pull Request:

1. On a test site running the latest WP 7.0 RC, create a post with a subtitle, publish, and refresh the editor.
2. Note that your subtitle doesn't show initially; try editing it and note it appears.
3. Apply this PR, and refresh the editor. Confirm that the subtitle now shows on load, and updates as usual when you change the text.
4. Smoketest the PR on a site running WP 6.9, just in case.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[Copilot]

Pull request overview

Adds a timed retry mechanism to ensure an existing post subtitle is appended beneath the post title in the (WP 7.0+) iframed editor, addressing cases where the title wrapper isn’t available immediately on load.

Changes:

• Extend appendSubtitleToTitleDOMElement with a bounded retry loop (100ms intervals, up to 20 attempts) when the title wrapper element isn’t yet present in the editor canvas.
• Update JSDoc for the DOM-append helper to include the internal retry counter parameter.

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