Add two new middlewares to improve robustness of the agent pipeline:
- DanglingToolCallMiddleware injects placeholder ToolMessages for
interrupted tool calls, preventing LLM errors from malformed history
- SubagentLimitMiddleware truncates excess parallel task tool calls at
the model response level, replacing the runtime check in task_tool
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
