fix: error seen when deleting a component used in a container (requires API change) [FC-0117]

[bradenmacdonald]

Description

Fixes openedx/frontend-app-authoring#3045

Deleting the draft version of a published component used in a published container would result in an error when LibraryXBlockMetadata tries to get info about the draft version.

# Event:  LIBRARY_BLOCK_DELETED
2026-05-07 00:18:03,180 INFO 1088 [openedx.core.djangoapps.content.search.tasks:157] - Updating content index documents for container lct:OpenCraftX:MTL3:unit:conclusion in librarylib:OpenCraftX:MTL3
2026-05-07 00:18:03,268 ERROR 1088 [celery_utils.logged_task:48] - failed due to Traceback (most recent call last):
  File "/openedx/edx-platform/openedx/core/djangoapps/content/search/tasks.py", line 159, in update_library_container_index_doc
    api.upsert_library_container_index_doc(container_key)
  File "/openedx/edx-platform/openedx/core/djangoapps/content/search/api.py", line 999, in upsert_library_container_index_doc
    """
        
  File "/openedx/edx-platform/openedx/core/djangoapps/content/search/documents.py", line 675, in searchable_doc_for_container
    published_children = lib_api.get_container_children(
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/openedx/edx-platform/openedx/core/djangoapps/content_libraries/api/containers.py", line 214, in get_container_children
    result.append(LibraryXBlockMetadata.from_component(container_key.lib_key, entry.entity.component))
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/openedx/edx-platform/openedx/core/djangoapps/content_libraries/api/block_metadata.py", line 70, in from_component
    display_name=draft.title,
                 ^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'title'

Unfortunately, fixing this properly requires completely changing how the get_container_children API works. The problem with the current version is that it's supposed to return either the draft or the published children, but it's returning the LibraryXBlockMetadata data structure that is designed to hold both versions of a given component, and assumes the draft version always exists.

To make this fix compatible with Verawood and avoid last-minute breaking changes to the API, I've just marked the broken function as deprecated and added a new _v2 function that doesn't have the same problem. This is a bit ugly but I think it's the safest approach.

Testing instructions

1. Create a component and a unit. Add the component to the unit.
2. Publish both the unit and the component.
3. Delete the component.
4. Without this fix, the component gets deleted but an error occurs when updating the container's search index entry. The traceback shown above will appear in the CMS logs, and the "child component count" on the parent container will be wrong (not decreased).

TODO

Add new tests / update tests as needed.

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