fix: truncate rwfx 3q context

offline-backend-20260512/backend/packages/harness/deerflow/agents/middlewares/task_external_context_middleware.py

@@ -17,6 +17,17 @@ from langchain_core.messages import HumanMessage
 
 _CONTEXT_KEY = "task_external_context"
 _TAG = "task_external_context"
+_MAX_CONTEXT_CHARS = 18000
+_TAIL_CONTEXT_CHARS = 2500
+
+
+def _truncate_middle(text: str, max_chars: int = _MAX_CONTEXT_CHARS) -> str:
+    if len(text) <= max_chars:
+        return text
+    marker = "\n\n...[隐藏任务上下文过长，已截断中间部分]...\n\n"
+    tail_chars = min(_TAIL_CONTEXT_CHARS, max_chars // 4)
+    head_chars = max(0, max_chars - len(marker) - tail_chars)
+    return f"{text[:head_chars]}{marker}{text[-tail_chars:]}"
 
 
 def _render_context(value: Any) -> str:
@@ -25,9 +36,10 @@ def _render_context(value: Any) -> str:
     if isinstance(value, str):
         return value.strip()
     try:
-        return json.dumps(value, ensure_ascii=False, indent=2, default=str).strip()
+        text = json.dumps(value, ensure_ascii=False, indent=2, default=str).strip()
     except Exception:
-        return str(value).strip()
+        text = str(value).strip()
+    return _truncate_middle(text)
 
 
 def _task_context_from_request(request: Any) -> Any: