Create papertrail versions for newly created/updated add_on_purchases

What does this MR do and why?

The GitlabSubscriptions::Duo::BulkAssignService uses upsert_all to create addon assignments, but this bypasses ActiveRecord callbacks which are required by papertrail to create version records. This results in orphaned addon assignments without corresponding subscription_user_add_on_assignment_versions records, causing data inconsistencies in ClickHouse historical data.

Example of the insert query and the query plan:

=== SQL ===
INSERT INTO subscription_user_add_on_assignment_versions (organization_id, item_id, purchase_id, user_id, created_at, item_type, event, namespace_path, add_on_name, whodunnit, object) VALUES (1030, 290, 41, 114, '2025-09-04 10:18:20.192383', 'GitlabSubscriptions::UserAddOnAssignment', 'create', '1030/', 'code_suggestions', NULL, '{"id":290,"add_on_purchase_id":41,"user_id":114,"created_at":"2025-09-04T10:18:20.192+00:00","updated_at":"2025-09-04T10:18:20.192+00:00","organization_id":1030}'), (1030, 291, 41, 113, '2025-09-04 10:18:20.192383', 'GitlabSubscriptions::UserAddOnAssignment', 'create', '1030/', 'code_suggestions', NULL, '{"id":291,"add_on_purchase_id":41,"user_id":113,"created_at":"2025-09-04T10:18:20.192+00:00","updated_at":"2025-09-04T10:18:20.192+00:00","organization_id":1030}')
.=== QUERY PLAN ===
                                        QUERY PLAN
------------------------------------------------------------------------------------------
 Insert on subscription_user_add_on_assignment_versions  (cost=0.00..0.01 rows=0 width=0)
   ->  Result  (cost=0.00..0.01 rows=1 width=240)

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Related to #566532 (closed)