Send page thumbnails, and null when the image is gone

[jeffreyvr]

Summary

• Push the featured image as thumbnail (https, large size) so search and chat cards can show it.
• Always send the key, including null, so removing a featured image and syncing again clears the stored URL in Datalumo.
• WooCommerce products still fall back to the product image when the post has no featured image of its own.

Test plan

• Sync a post with a featured image and confirm the page thumbnail is set in Datalumo.
• Remove that featured image, sync again, and confirm the thumbnail clears.
• Sync a Woo product with no featured image but a product image and confirm that URL is sent.
• composer test in the plugin directory.

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