[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/memories/write/src/runtime.rs

@@ -261,6 +261,7 @@ impl MemoryStartupContext {
             config.features.enabled(Feature::RuntimeMetrics),
             /*beta_features_header*/ None,
             config.features.enabled(Feature::ItemIds),
+            /*concurrent_reasoning_summaries_enabled*/ false,
             /*attestation_provider*/ None,
         );