Agent 循环单次迭代 / One Agent Loop Iteration

说明（zh）：一次迭代以 query() 为轴心：组装消息、调用模型、解析出文本与 tool_use；若有工具则校验、权限检查、执行并写回 tool_result，再进入下一轮；否则向用户返回并终止。

Notes (en): Each iteration centers on query(): build messages, call the model, parse text and tool_use blocks; if tools run, validate, check permissions, execute, append tool_result, and continue; otherwise return to the user and stop.