Persist tokens to disk, preserving any stored client info.
Also records the absolute Unix-epoch expiry as a sidecar so a
cold-started provider can detect a stale access token and trigger
the SDK's refresh_token grant instead of a full browser re-auth.
Cleared when expires_in is absent so the sidecar can't go stale.
Once persistence starts, cancellation is delayed until the write is terminal. If persistence fails while cancellation is pending, the persistence error takes precedence so it is not silently discarded.
