Codex round-1 SHOULD-FIX: the PR's only test change mocked the new write to a
no-op. Add real-SessionDB tests proving generated-title persistence, manual-rename
no-overwrite, and cli/gateway source preservation on ensure_session. Additive
test-only; no behavior change to the contributor's fix.
Co-authored-by: liuguangyong93 <liuguangyong93@users.noreply.github.com>