Media cannot be removed via context menu in metadata editor

[solth]

Describe the bug
Deleting media via the context menu option of the structure tree and gallery of the metadata editor does not remove the media anymore.

To Reproduce
Steps to reproduce the behavior:

1. Open a process with media in the metadata editor
2. Open the context menu for a node in the structure tree representing a media element (e.g. a page or track)
3. Click the option "Delete media" in the context menu
4. See that media is not removed

Expected behavior
Deleting media via the context menu of the structure tree of gallery in the metadata editor should work

Screenshots

Bildschirmaufnahme.2025-09-25.um.10.38.41.mp4

Release
3.9.0.RC1, current main branch

Additional context
Deleting media via the context menu does work in Kitodo 3.8.x and presumably has been broken by the integration of hibernate-search. 

[solth]

It seems this wasn't caused by the integration of HibernateSearch after all, but by the changes to the metadata editor introduced in #6340

Apparently the variable StructurePanel.selectedPhysicalNodes, which is used to hold the media that should be removed, is not set correctly in case of the unified structure tree anymore, hence trying to remove them does not work (effectively the method deletes the contents of an empty array)

[solth]

@thomaslow would you mind taking a look at this? Since you heavily refactored the metadata editor code in #6340 you probably know best how to tackle this issue?