Editor: Closing a post with an ongoing media upload in Cover block results in malformed content

Expected behavior

Closing a post with ongoing media uploads should produce valid content.

Actual behavior

Closing a post with ongoing media uploads produces malformed content.

Steps to reproduce the behavior

[!NOTE]
It's recommended to use the Network Link Conditioner to ensure the upload duration allows you to close the editor before finishes.

1. Add a Cover block and tap the placeholder.
2. Select "Choose from device" option.
3. Select an image from the device and confirm the selection.
4. While the image is uploading, leave the editor.
5. Verify that you see the Uploading media... placeholder in the post summary.
6. Wait for the upload to complete while in the post list.
7. Re-open the post with the Cover block.
8. Observe the Cover block is rendered with the error Problem displaying block.

https://github.com/wordpress-mobile/WordPress-iOS/assets/14905380/fb4cfdf4-3520-4568-af1c-466403527be4

Tested on iPhone 11, iOS 17.0.2, Jetpack iOS / WordPress iOS 24.5.0.1

Start by reproducing the Cover block workflow: begin an image upload, leave the editor, wait for completion, and reopen the post. Trace the Cover block's upload and post-saving handling to find why completed uploads produce malformed content; done means the reopened block renders normally without "Problem displaying block".