test(engine): let a start on a cancelled parent lose the race to its child

child_wall_time_is_unbounded_unless_an_owner_sets_it cancels the parent's token
on purpose so no provider request runs. run_prepared_subagent_task selects on
that token first, so a child that is polled before AgentTool start commits its
launch exits and drops the receiver, and start reports "prepared child
execution owner closed". That is the intended behavior for a cancelled parent,
but the test unwrapped the result and failed whenever a yield let the child run
first (blocked #7211, #7217, #7218, #7228, #7229 on ubuntu and windows). The
test now retries that one error up to 50 times and fails on any other error.

Verified: the test passed 25 of 25 consecutive runs after the change. It was not
run in a loop before the change, so the prior rate is unmeasured here. No hosted
CI.

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

crates/engine/src/tools/subagent/limits_tests.rs

@@ -353,21 +353,33 @@ async fn child_wall_time_is_unbounded_unless_an_owner_sets_it() {
     drop(guard);
     for owner_steps in [0, 7] {
         runtime.worker_profile.max_steps = owner_steps;
-        let result = AgentTool::new(Arc::clone(&manager), runtime.clone())
-            .execute(
-                json!({
-                    "action": "start",
-                    "type": "explore",
-                    "prompt": "inspect",
-                    "max_steps": 1,
-                    "wall_time_secs": 1,
-                    "max_output_tokens": 1,
-                    "max_depth": 0
-                }),
-                &runtime.context,
-            )
-            .await
-            .expect("legacy model caps cannot restrict the child");
+        let args = json!({
+            "action": "start",
+            "type": "explore",
+            "prompt": "inspect",
+            "max_steps": 1,
+            "wall_time_secs": 1,
+            "max_output_tokens": 1,
+            "max_depth": 0
+        });
+        let mut closed_before_commit = 0;
+        let result = loop {
+            match AgentTool::new(Arc::clone(&manager), runtime.clone())
+                .execute(args.clone(), &runtime.context)
+                .await
+            {
+                Ok(result) => break result,
+                Err(error)
+                    if closed_before_commit < 50
+                        && error
+                            .to_string()
+                            .contains("prepared child execution owner closed") =>
+                {
+                    closed_before_commit += 1;
+                }
+                Err(error) => panic!("legacy model caps cannot restrict the child: {error}"),
+            }
+        };
         let receipt: Value = serde_json::from_str(&result.content).unwrap();
         assert_eq!(receipt["effective_limits"]["max_steps"], owner_steps);
         assert!(receipt["effective_limits"]["wall_deadline_ms"].is_null());