feat(core, tracing): add a span representing a turn (#13424)

This is PR 3 of the app-server tracing rollout.

PRs https://github.com/openai/codex/pull/13285 and
https://github.com/openai/codex/pull/13368 gave us inbound request spans
in app-server and propagated trace context through Submission. This
change finishes the next piece in core: when a request actually starts a
turn, we now create a core-owned long-lived span that stays open for the
real lifetime of the turn.

What changed:
- `Session::spawn_task` can now optionally create a long-lived turn span
and run the spawned task inside it
- `turn/start` uses that path, so normal turn execution stays under a
single core-owned span after the async handoff
- `review/start` uses the same pattern
- added a unit test that verifies the spawned turn task inherits the
submission dispatch trace ancestry

**Why**
The app-server request span is intentionally short-lived. Once work
crosses into core, we still want one span that covers the actual
execution window until completion or interruption. This keeps that
ownership where it belongs: in the layer that owns the runtime
lifecycle.

codex-rs/core/src/tasks/compact.rs

@@ -17,6 +17,10 @@ impl SessionTask for CompactTask {
         TaskKind::Compact
     }
 
+    fn span_name(&self) -> &'static str {
+        "session_task.compact"
+    }
+
     async fn run(
         self: Arc<Self>,
         session: Arc<SessionTaskContext>,

codex-rs/core/src/tasks/ghost_snapshot.rs

@@ -32,6 +32,10 @@ impl SessionTask for GhostSnapshotTask {
         TaskKind::Regular
     }
 
+    fn span_name(&self) -> &'static str {
+        "session_task.ghost_snapshot"
+    }
+
     async fn run(
         self: Arc<Self>,
         session: Arc<SessionTaskContext>,

codex-rs/core/src/tasks/mod.rs

@@ -14,7 +14,7 @@ use tokio::sync::Notify;
 use tokio_util::sync::CancellationToken;
 use tokio_util::task::AbortOnDropHandle;
 use tracing::Instrument;
-use tracing::Span;
+use tracing::info_span;
 use tracing::trace;
 use tracing::warn;
 
@@ -89,6 +89,9 @@ pub(crate) trait SessionTask: Send + Sync + 'static {
     /// surface it in telemetry and UI.
     fn kind(&self) -> TaskKind;
 
+    /// Returns the tracing name for a spawned task span.
+    fn span_name(&self) -> &'static str;
+
     /// Executes the task until completion or cancellation.
     ///
     /// Implementations typically stream protocol events using `session` and
@@ -127,6 +130,7 @@ impl Session {
 
         let task: Arc<dyn SessionTask> = Arc::new(task);
         let task_kind = task.kind();
+        let span_name = task.span_name();
 
         let cancellation_token = CancellationToken::new();
         let done = Arc::new(Notify::new());
@@ -137,7 +141,15 @@ impl Session {
             let ctx = Arc::clone(&turn_context);
             let task_for_run = Arc::clone(&task);
             let task_cancellation_token = cancellation_token.child_token();
-            let session_span = Span::current();
+            // Task-owned turn spans keep a core-owned span open for the
+            // full task lifecycle after the submission dispatch span ends.
+            let task_span = info_span!(
+                "turn",
+                otel.name = span_name,
+                thread.id = %self.conversation_id,
+                turn.id = %turn_context.sub_id,
+                model = %turn_context.model_info.slug,
+            );
             tokio::spawn(
                 async move {
                     let ctx_for_finish = Arc::clone(&ctx);
@@ -158,7 +170,7 @@ impl Session {
                     }
                     done_clone.notify_waiters();
                 }
-                .instrument(session_span),
+                .instrument(task_span),
             )
         };
 

codex-rs/core/src/tasks/regular.rs

@@ -68,6 +68,10 @@ impl SessionTask for RegularTask {
         TaskKind::Regular
     }
 
+    fn span_name(&self) -> &'static str {
+        "session_task.turn"
+    }
+
     async fn run(
         self: Arc<Self>,
         session: Arc<SessionTaskContext>,

codex-rs/core/src/tasks/review.rs

@@ -43,6 +43,10 @@ impl SessionTask for ReviewTask {
         TaskKind::Review
     }
 
+    fn span_name(&self) -> &'static str {
+        "session_task.review"
+    }
+
     async fn run(
         self: Arc<Self>,
         session: Arc<SessionTaskContext>,

codex-rs/core/src/tasks/undo.rs

@@ -31,6 +31,10 @@ impl SessionTask for UndoTask {
         TaskKind::Regular
     }
 
+    fn span_name(&self) -> &'static str {
+        "session_task.undo"
+    }
+
     async fn run(
         self: Arc<Self>,
         session: Arc<SessionTaskContext>,

codex-rs/core/src/tasks/user_shell.rs

@@ -66,6 +66,10 @@ impl SessionTask for UserShellCommandTask {
         TaskKind::Regular
     }
 
+    fn span_name(&self) -> &'static str {
+        "session_task.user_shell"
+    }
+
     async fn run(
         self: Arc<Self>,
         session: Arc<SessionTaskContext>,