Capture rollout budget units from response usage (#36641)

## What changed

- Parse `codex_rollout_budget_units` from completed Responses API usage into
  `TokenUsage`.
- Keep the provider-only value out of serialized protocol, JSON schema, and
  TypeScript representations.

## Testing

- Cover the value in unit and end-to-end SSE response parsing tests.

GitOrigin-RevId: a53d4202beb9c8985d25894cb10e7c01f20b2a44

codex-rs/ext/goal/src/accounting.rs

@@ -324,6 +324,7 @@ fn token_delta_since_last_accounting(last: &TokenUsage, current: &TokenUsage) ->
             .reasoning_output_tokens
             .saturating_sub(last.reasoning_output_tokens),
         total_tokens: current.total_tokens.saturating_sub(last.total_tokens),
+        codex_rollout_budget_units: None,
     };
     goal_token_delta_for_usage(&delta)
 }