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/analytics/src/analytics_client_tests.rs

@@ -389,6 +389,7 @@ fn sample_turn_token_usage_fact(thread_id: &str, turn_id: &str) -> TurnTokenUsag
             cache_write_input_tokens: 7,
             output_tokens: 140,
             reasoning_output_tokens: 13,
+            codex_rollout_budget_units: None,
         },
     }
 }