Address review feedback on #63068:
1. Replace unconditional reopen_session() + end_session() with a
conditional promote_to_session_reset() method in SessionDB.
The new method only promotes live rows or rows ended with
agent_close — explicit boundaries (compression, session_reset,
new_command) are preserved via first-writer-wins semantics.
2. Rewrite tests to use real SessionDB instead of MagicMock:
- 7 unit tests for promote_to_session_reset edge cases
- 3 integration tests verifying the actual recovery contract
in find_latest_gateway_session_for_peer after promotion
saitsuki
3305dcedbb