feat: implement graceful tool cancellation on stop request

- Add stop flag monitoring loop (checks every 100ms during tool execution)
- Cancel tool task immediately when stop flag is detected
- Return cancellation message to conversation history with role=tool
- Save cancellation result: '命令执行被用户取消'
- Clean up pending tasks to prevent 'Task was destroyed but it is pending' warnings
- Fix terminal_ops.py to properly cancel stdout/stderr read tasks

Known issue: Tool result display in frontend still shows arguments instead of cancellation message when expanded

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

modules/terminal_ops.py

@@ -537,6 +537,13 @@ class TerminalOperator:
                         process.kill()
                     except Exception:
                         pass
+                # 取消读取任务，避免孤儿任务
+                stdout_task.cancel()
+                stderr_task.cancel()
+                try:
+                    await asyncio.gather(stdout_task, stderr_task, return_exceptions=True)
+                except Exception:
+                    pass
                 raise
 
             # 确保读取协程结束

server/chat_flow_tool_loop.py

@@ -484,3 +484,5 @@ async def execute_tool_calls(*, web_terminal, tool_calls, sender, messages, clie
 
 
     return {"stopped": False, "last_tool_call_time": last_tool_call_time}
+
+