Replay pending deletions on QueueManager.Initialize

[stidsborg]

Summary

• DeliveredPositionsId was migrated in d8fefad from a parent-with-children layout to a single List<long> value, but QueueManager.Initialize still scanned for children — which no writer produces anymore.
• Restarted flows therefore never replayed pending message deletions: messages already captured into delivery effects could remain in the message store indefinitely.
• Read the persisted List<long> directly, matching the AfterFlush write pattern (delete from store, clear in place, flushless-upsert empty list).

Test plan

• dotnet build core project
• In-memory messaging test suite (56/56 pass)
• Store-backed messaging suites (Postgres / SQL Server / MariaDB) on CI