feat: add DanglingToolCallMiddleware and SubagentLimitMiddleware

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>

backend/src/tools/builtins/task_tool.py

@@ -11,7 +11,7 @@ from langgraph.typing import ContextT
 
 from src.agents.thread_state import ThreadState
 from src.subagents import SubagentExecutor, get_subagent_config
-from src.subagents.executor import MAX_CONCURRENT_SUBAGENTS, SubagentStatus, count_active_tasks_by_trace, get_background_task_result
+from src.subagents.executor import SubagentStatus, get_background_task_result
 
 logger = logging.getLogger(__name__)
 
@@ -86,11 +86,6 @@ def task_tool(
         # Get or generate trace_id for distributed tracing
         trace_id = metadata.get("trace_id") or str(uuid.uuid4())[:8]
 
-    # Check sub-agent limit before creating a new one
-    if trace_id and count_active_tasks_by_trace(trace_id) >= MAX_CONCURRENT_SUBAGENTS:
-        logger.warning(f"[trace={trace_id}] Sub-agent limit reached ({MAX_CONCURRENT_SUBAGENTS}). Rejecting new task: {description}")
-        return f"Error: Maximum number of concurrent sub-agents ({MAX_CONCURRENT_SUBAGENTS}) reached. Please wait for existing tasks to complete before launching new ones."
-
     # Get available tools (excluding task tool to prevent nesting)
     # Lazy import to avoid circular dependency
     from src.tools import get_available_tools