fix(session): copy compactionPoints with correct ID mapping (#538)

* feat(session): add copyMessagesWithMapping helper for ID tracking

* feat(session): update copyThreads to map compactionPoints IDs

* fix(session): map compactionPoints IDs when copying session

* test(session): add unit tests for compactionPoints copying

* fix(session): address PR review - support compactionPoints in sourceMeta

- Remove unused copyMessage import
- Add compactionPoints to copySession sourceMeta type
- Fix moveThreadToConversations to pass thread's compactionPoints
- Add test case for external idMapping parameter