fix(content_libraries): keep collection num_children in sync after item updates

[kingoftech-v01]

Description

When adding or removing items from a library collection, the search index's num_children count was stale. Users saw incorrect counts on collection cards until a later unrelated edit triggered another reindex.

User impact (Library Author): Collection counts in the Studio library view reflect the actual number of items immediately after add/remove operations.

Root Cause

update_library_collection_items (previously update_library_collection_components) relied on Django's Collection.post_save signal to emit LIBRARY_COLLECTION_UPDATED. That signal fires before the M2M entities relationship commits. The synchronous reindex handler (search/handlers.py:234) then runs searchable_doc_for_collection, which computes num_children = filter_publishable_entities(collection.entities, has_draft=True).count() against the stale pre-commit M2M state. The final count is never corrected because the m2m_changed handler only emits CONTENT_OBJECT_ASSOCIATIONS_CHANGED per item, not LIBRARY_COLLECTION_UPDATED.

Git history:

• PR feat: Add Library Collections REST endpoints [FC-0062] #35321 (7665f13547, 2024-09-11, Yusuf Musleh) originally had the explicit emission.
• PR Soft delete collections #35496 (5446877a86, 2024-09-26, Jillian Vogel) refactored to post_save signals and removed the explicit emit.
• PR fix: Keep sync component count in collection card [FC-0062] #35734 (d5850c812c, 2024-11-19, Chris Chávez) — was merged (contrary to the bug report label) and restored the pattern in delete_library_block, revert_changes, and later delete_container / restore_container. update_library_collection_items was left as the lone outlier.

Fix

Two surgical changes:

1. openedx/core/djangoapps/content_libraries/api/collections.py — emit LIBRARY_COLLECTION_UPDATED with background=True explicitly after add_to_collection / remove_from_collection. The background=True flag routes the reindex to a Celery task that runs post-transaction, after the M2M commit, so it sees the correct count. This mirrors set_library_item_collections, delete_library_block, restore_library_block, delete_container, and restore_container.

2. openedx/core/djangoapps/content_libraries/rest_api/collections.py — decorate LibraryCollectionsView with @method_decorator(transaction.non_atomic_requests, name="dispatch"), matching every other view in the same subpackage (blocks.py, libraries.py, containers.py). Without this, a Celery .delay() dispatch from inside the per-request atomic transaction could be consumed before the transaction commits.

Supporting Information

• Issue: Collection count and contents are sometimes incorrect #35776
• Original emission PR: feat: Add Library Collections REST endpoints [FC-0062] #35321 (7665f13547)
• Refactor that removed it: Soft delete collections #35496 (5446877a86)
• Sibling fixes: fix: Keep sync component count in collection card [FC-0062] #35734 (d5850c812c) and later containers work
• Reference correct pattern: set_library_item_collections (collections.py:226-239)

Testing Instructions

1. In Studio, create a content library and a collection.
2. Add items to the collection.
3. Verify the collection card shows the correct item count immediately (before the fix it would show a stale count).
4. Remove items. Verify the count decrements immediately.
5. Confirm search/index consistency: searchable_doc_for_collection(collection_key)["num_children"] matches collection.entities.count().

Two new/updated tests in openedx/core/djangoapps/content_libraries/tests/test_api.py:

• test_update_library_collection_components_event — updated expectation from 4 to 5 events and added a final-call assertion verifying the background=True emit.
• test_bug_35776_regression_update_items_emits_background_updated — regression test covering both add and remove paths, asserting background=True emissions.

Deadline

None

Other Information

• No database migrations
• No new dependencies
• No deprecations
• Emits two LIBRARY_COLLECTION_UPDATED per update_items call (one from post_save, one explicit after M2M) — the second overrides the first, final state is correct. Matches the behavior already accepted for set_library_item_collections.
• non_atomic_requests decorator matches every other view in content_libraries/rest_api/. The sub-operations in the view are already individually atomic where needed.

Closes #35776

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