Fix(subagent): Event loop conflict in SubagentExecutor.execute() (#1965)

* Fix event loop conflict in SubagentExecutor.execute()

When SubagentExecutor.execute() is called from within an already-running
event loop (e.g., when the parent agent uses async/await), calling
asyncio.run() creates a new event loop that conflicts with asyncio
primitives (like httpx.AsyncClient) that were created in and bound to
the parent loop.

This fix detects if we're already in a running event loop, and if so,
runs the subagent in a separate thread with its own isolated event loop
to avoid conflicts.

Fixes: sub-task cards not appearing in Ultra mode when using async parent agents

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(subagent): harden isolated event loop execution

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tests/test_subagent_executor.py

@@ -433,6 +433,42 @@ class TestSyncExecutionPath:
         assert result.status == SubagentStatus.COMPLETED
         assert result.result == "Thread pool result"
 
+    @pytest.mark.anyio
+    async def test_execute_in_running_event_loop_uses_isolated_thread(self, classes, base_config, mock_agent, msg):
+        """Test that execute() uses the isolated-thread path inside a running loop."""
+        SubagentExecutor = classes["SubagentExecutor"]
+        SubagentStatus = classes["SubagentStatus"]
+
+        execution_threads = []
+        final_state = {
+            "messages": [
+                msg.human("Task"),
+                msg.ai("Async loop result", "msg-1"),
+            ]
+        }
+
+        async def mock_astream(*args, **kwargs):
+            execution_threads.append(threading.current_thread().name)
+            yield final_state
+
+        mock_agent.astream = mock_astream
+
+        executor = SubagentExecutor(
+            config=base_config,
+            tools=[],
+            thread_id="test-thread",
+        )
+
+        with patch.object(executor, "_create_agent", return_value=mock_agent):
+            with patch.object(executor, "_execute_in_isolated_loop", wraps=executor._execute_in_isolated_loop) as isolated:
+                result = executor.execute("Task")
+
+        assert isolated.call_count == 1
+        assert execution_threads
+        assert all(name.startswith("subagent-isolated-") for name in execution_threads)
+        assert result.status == SubagentStatus.COMPLETED
+        assert result.result == "Async loop result"
+
     def test_execute_handles_asyncio_run_failure(self, classes, base_config):
         """Test handling when asyncio.run() itself fails."""
         SubagentExecutor = classes["SubagentExecutor"]