codex exec --json omits usage and a typed code on rollout-budget exhaustion

Summary

When the experimental native features.rollout_budget stops a codex exec --json turn, Codex emits a turn.failed event containing only the English message shared rollout token budget exhausted. The event has no usage object or machine-readable error code, even though the internal rollout meter necessarily had a value to make the decision.

This makes it possible for an orchestrator to recognize the limit only by matching prose, and impossible to persist the usage that was available inside Codex at termination.

Reproduction

codex exec --json --ephemeral --ignore-user-config --ignore-rules \
  --skip-git-repo-check \
  -c 'features.rollout_budget={enabled=true,limit_tokens=1,reminder_at_remaining_tokens=[],sampling_token_weight=1.0,prefill_token_weight=1.0}' \\\n  'Reply with exactly: ok'\n```\n\n### Observed JSONL\n\n```jsonl\n{"type":"thread.started","thread_id":"<redacted>"}\n{"type":"item.completed","item":{"id":"item_0","type":"error","message":"Under-development features enabled: rollout_budget. ..."}}\n{"type":"turn.started"}\n{"type":"item.completed","item":{"id":"item_1","type":"agent_message","text":"ok"}}\n{"type":"error","message":"shared rollout token budget exhausted"}\n{"type":"turn.failed","error":{"message":"shared rollout token budget exhausted"}}\n```\n\nThe process exits 1. There is no `usage` on the terminal event.\n\n### Expected\n\nThe terminal event should expose:\n\n- a stable machine-readable kind/code such as `rollout_budget_exhausted`; and\n- the final rollout-budget units and available token usage used for the enforcement decision (while preserving the existing message for humans).\n\nA dedicated meter value is important because rollout-budget units are not necessarily portable input+output tokens: Codex can use provider-reported `codex_rollout_budget_units`, otherwise its configured weighted fallback.\n\n### Environment\n\n- `codex-cli 0.145.0`\n- macOS Darwin 25.6.0 arm64\n\nRelated but distinct from #37138: that issue covers a provider response missing usage causing the budget not to advance; this report covers the JSONL terminal after the native budget does advance and stops the turn.