fix: add block theme check before switching templates to prevent warnings

[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 change just gets rid of some annoying notices when you're using the Block theme:

[21-Jan-2026 21:15:22 UTC] PHP Warning: include(): Failed opening '/var/www/additional-sites-html/blocktheme2/wp-content/themes/newspack-block-theme/single-wide.php' for inclusion (include_path='.:/usr/share/php') in /var/www/additional-sites-html/blocktheme2/wp-includes/template-loader.php on line 125

How to test the changes in this Pull Request:

1. Apply the PR.
2. Using the Classic Theme, edit your /cart page and make sure it's set to use the Default or One-Column (not wide) template.
3. Ensure it's still using the One-Column wide template on the front-end.

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?