feat: expose --reconnection-grace-time CLI flag (#7678)

* feat: expose --reconnection-grace-time CLI flag

Pass through VS Code Server's --reconnection-grace-time argument,
allowing users to configure how long the server waits for a
disconnected client to reconnect before cleaning up the session.

This is useful for users whose client machines sleep overnight, causing
the default 3-hour grace period to expire and forcing a "Reload Window"
on wake. The flag can also be set via CODE_SERVER_RECONNECTION_GRACE_TIME
env var or in config.yaml.