fork_session passed each source message's metadata dict by reference into the
new session. add_message() -> _persist_message() stamps _db_id (and timestamp)
onto that dict in place, so persisting the fork overwrote the SOURCE messages'
_db_id with the forked rows' ids — silently breaking edit/delete-by-id on the
original conversation.
Copy the metadata dict per message so the fork and source no longer alias.
Adds tests/test_fork_session_metadata.py asserting the source session's
message metadata is unchanged after a fork.
Mazen Tamer Salah