fix: queue concurrent subagents (#5566)

* fix: queue concurrent subagents

* chore: keep spawn schema concise

tests/agent/test_subagent_lifecycle.py

@@ -520,6 +520,35 @@ class TestCancelBySession:
         count = await sm.cancel_by_session("nonexistent")
         assert count == 0
 
+    @pytest.mark.asyncio
+    async def test_cancels_active_and_queued_tasks(self, tmp_path):
+        sm = _manager(tmp_path, max_concurrent_subagents=1)
+        active_entered = asyncio.Event()
+        queued_entered = asyncio.Event()
+
+        async def _blocked_run(spec):
+            task = spec.initial_messages[-1]["content"]
+            if task == "active":
+                active_entered.set()
+            else:
+                queued_entered.set()
+            await asyncio.Event().wait()
+
+        sm.runner.run = _blocked_run
+        runtime = _runtime()
+        await sm.spawn("active", runtime=runtime, session_key="s1")
+        await asyncio.wait_for(active_entered.wait(), timeout=1.0)
+        await sm.spawn("queued", runtime=runtime, session_key="s1")
+        await asyncio.sleep(0)
+
+        assert not queued_entered.is_set()
+        assert await sm.cancel_by_session("s1") == 2
+        await asyncio.sleep(0)
+
+        assert not queued_entered.is_set()
+        assert sm._running_tasks == {}
+        assert sm._session_tasks == {}
+
     @pytest.mark.asyncio
     async def test_already_done_not_counted(self, tmp_path):
         sm = _manager(tmp_path)