[codex] Support sequential cutoff reasoning summaries (#31306)

## Summary

When enabled for the OpenAI provider, Codex sends
`stream_options.reasoning_summary_delivery = "sequential_cutoff"` on
HTTP and
WebSocket requests, including prewarm, and renders completed summary
sections
from `reasoning_summary_text.done`. Flag-off and non-OpenAI behavior is
unchanged.

## Expected rollout

```text
reasoning 0 added
summary 0 done
summary 1 done
summary 2 starts
summary 2 cancelled / incomplete
reasoning 0 done  <-- cancel summary 2 work and mark it incomplete
message 1 added
message 1 text streams
message 1 completed
```

Depends on
[openai/openai#1096660](https://github.com/openai/openai/pull/1096660).

codex-rs/codex-api/src/endpoint/responses_websocket.rs

@@ -851,6 +851,7 @@ mod tests {
             reasoning: None,
             store: false,
             stream: true,
+            stream_options: None,
             include: vec!["reasoning.encrypted_content".to_string()],
             service_tier: Some("priority".to_string()),
             prompt_cache_key: Some("cache-key".to_string()),