[rollout_trace] Record core session rollout traces (#18877)

## Summary

Wires rollout trace recording into `codex-core` session and turn
execution. This records the core model request/response, compaction, and
session lifecycle boundaries needed for replay without yet tracing every
nested runtime/tool boundary.

## Stack

This is PR 2/5 in the rollout trace stack.

- [#18876](https://github.com/openai/codex/pull/18876): Add rollout
trace crate
- [#18877](https://github.com/openai/codex/pull/18877): Record core
session rollout traces
- [#18878](https://github.com/openai/codex/pull/18878): Trace tool and
code-mode boundaries
- [#18879](https://github.com/openai/codex/pull/18879): Trace sessions
and multi-agent edges
- [#18880](https://github.com/openai/codex/pull/18880): Add debug trace
reduction command

## Review Notes

This layer is the first live integration point. The important review
question is whether trace recording is isolated from normal session
behavior: trace failures should not become user-visible execution
failures, and recording should preserve the existing turn/session
lifecycle semantics.

The PR depends on the reducer/data model from the first stack entry and
only introduces the core recorder surface that later PRs use for richer
runtime and relationship events.

codex-rs/core/tests/responses_headers.rs

@@ -131,6 +131,7 @@ async fn responses_stream_includes_subagent_header_on_review() {
             summary.unwrap_or(model_info.default_reasoning_summary),
             /*service_tier*/ None,
             /*turn_metadata_header*/ None,
+            &codex_rollout_trace::InferenceTraceContext::disabled(),
         )
         .await
         .expect("stream failed");
@@ -257,6 +258,7 @@ async fn responses_stream_includes_subagent_header_on_other() {
             summary.unwrap_or(model_info.default_reasoning_summary),
             /*service_tier*/ None,
             /*turn_metadata_header*/ None,
+            &codex_rollout_trace::InferenceTraceContext::disabled(),
         )
         .await
         .expect("stream failed");
@@ -372,6 +374,7 @@ async fn responses_respects_model_info_overrides_from_config() {
             summary.unwrap_or(model_info.default_reasoning_summary),
             /*service_tier*/ None,
             /*turn_metadata_header*/ None,
+            &codex_rollout_trace::InferenceTraceContext::disabled(),
         )
         .await
         .expect("stream failed");