_enforce_grok_cost_budget killed + evicted the container without calling
_finalize_spawn_session, so the open agent_spawn_sessions row stayed open
(ended_at IS NULL) and the burned usage/cost was never recorded in the
dashboard.
Call _finalize_spawn_session(exit_reason='cost_cap') BEFORE popping the
instance — it reads self._instances[agent_id] for the model +
usage_session_id, which the pop would lose.