STAC collection thumbnail images can be defined but not edited

When I add a thumbnail image to a STAC collection, it gets added fine, but when opening that collection, the field is not filled and the property cannot be edited.

Steps to reproduce:
1. Open a collection without a thumbnail in STAC Manager
2. Add a thumbnail via the dedicated form fields
3. Save
4. Open the collection and confirm that the thumbnail shows up correctly
5. Verify on STAC API that the collection-level asset has been added correctly

6. Edit the collection
7. Note that the Thumbnail form fields are not filled and the added collection-level asset is not listed

8. Expected would be: The configured thumbnail shows up either in the collection-level assets or the dedicated form fields.

Start by reproducing the issue in STAC Manager and inspect the collection edit flow after saving a thumbnail. Trace how the collection-level asset is read into the edit form, then verify that the thumbnail appears in either the asset list or dedicated fields and remains editable; confirm the result against the STAC API.

Written by the indexing model from the issue text.