Make subagents async and reusable by default (#3887)

tests/agent_harness_raw_coverage_e2e.rs

@@ -588,6 +588,7 @@ inline = "Answer the delegated cache probe directly."
                     "agent_id": "cache_probe_child",
                     "prompt": "Inspect whether the child turn can answer a cache probe.",
                     "context": "Parent observed request id cache-42.",
+                    "blocking": true,
                 }),
             )],
             140,

tests/inference_agent_raw_coverage_e2e.rs

@@ -4698,6 +4698,7 @@ async fn agent_subagent_public_types_cover_task_local_and_error_display_paths()
         elapsed: Duration::from_millis(12),
         mode: SubagentMode::Typed,
         status: SubagentRunStatus::Completed,
+        final_history: Vec::new(),
     };
     assert_eq!(outcome.mode.as_str(), "typed");
     assert_eq!(outcome.elapsed.as_millis(), 12);