Cover block background settings update for use featured image.

[rinkalpagdar]

What problem does this address?

When editing a template (e.g., a single post template) in WordPress and adding a Cover block, the option to fix the background image (has_parallax) is available only when a static image is selected manually. However, if the Cover block is set to use the post’s Featured Image dynamically, the controls for fixing the image disappear entirely.

Steps to Reproduce:

• Edit a template (e.g., a Single Post template) in the Site Editor.
• Add a Cover block.
• Set a manual image → Notice that the "Fixed Background" (has_parallax) option is available.
• Switch the image source to "Use Featured Image" → The Fixed Background option disappears.
  The background remains fixed but cannot be adjusted afterwards.

What is your proposed solution?

The Fixed Background option should be available and functional when using a Featured Image as well.
Users should be able to toggle this setting regardless of whether the image is static or dynamic.

[hanneslsm]

Related (same?): #69179

[t-hamano]

Closing as a duplicate of #69179. The description seems to be exactly the same.