Track sub-agent result delivery outcomes (#51331)

## What changed

Increment `codex.multi_agent.result_delivery` when sending a terminal child result to its parent, with `outcome=queued` for queue acceptance and `outcome=failed` for delivery errors.

GitOrigin-RevId: 7571f66c2c2d2baea1a54dce3741f6393e809b66

codex-rs/core/src/agent/control/completion.rs

@@ -1,7 +1,7 @@
 //! Delivers terminal child results and completion activity to the agent tree.
 //!
 //! Sessions capture terminal state; the controller owns routing and queue-only delivery.
-//! Delivery remains best effort, with tracing recorded only after the parent accepts it.
+//! Delivery remains best effort; metrics distinguish queue acceptance from delivery failure.
 
 use super::LocalAgentControl;
 use crate::TurnStartOptions;
@@ -116,15 +116,25 @@ impl LocalAgentControl {
         );
         let context =
             AgentCommunicationContext::new(AgentCommunicationKind::Result, outcome.thread_id);
-        if let Err(err) = self
+        let delivery = self
             .send_inter_agent_communication(
                 parent_thread_id,
                 communication,
                 context,
                 TurnStartOptions::default(),
             )
-            .await
-        {
+            .await;
+        if let Some(metrics) = codex_otel::global() {
+            let _ = metrics.counter(
+                "codex.multi_agent.result_delivery",
+                /*inc*/ 1,
+                &[(
+                    "outcome",
+                    if delivery.is_ok() { "queued" } else { "failed" },
+                )],
+            );
+        }
+        if let Err(err) = delivery {
             debug!("failed to notify parent thread {parent_thread_id}: {err}");
             return;
         }