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/core/src/tasks/mod.rs

@@ -678,6 +678,7 @@ impl Session {
                 total_tokens: (total_token_usage.total_tokens
                     - token_usage_at_turn_start.total_tokens)
                     .max(0),
+                codex_rollout_budget_units: None,
             };
             let current_span = Span::current();
             current_span.record(