fix: put preempted Syscall(Executing) coroutines back in ready queue instead of syscall map

When SIGURG preempts a coroutine in Syscall(Executing) state (due to a
race between the monitor sending SIGURG and MonitorListener removing
the notify node), the scheduler was putting it in the syscall map
without a syscall_suspend entry. This caused the coroutine to get
stuck forever — no mechanism moves Syscall(Executing) coroutines from
the syscall map back to the ready queue.

Fix: detect Syscall(Executing) yields as preemption events and push
the coroutine back to the ready queue. resume_with() already accepts
Syscall(Executing) → Running transitions, so the coroutine resumes
correctly from inside the signal handler.

Agent-Logs-Url: https://github.com/acl-dev/open-coroutine/sessions/2b3f1622-04e3-4f08-945a-e1e391d32eb0

Co-authored-by: loongs-zhang <38336731+loongs-zhang@users.noreply.github.com>

Author: Copilot

core/src/scheduler.rs

@@ -311,12 +311,17 @@ impl<'s> Scheduler<'s> {
                     _ = RUNNING_COROUTINES.remove(&co_id);
                 })? {
                     CoroutineState::Syscall((), _, state) => {
-                        //挂起协程到系统调用表
-                        //如果已包含，说明当前系统调用还有上层父系统调用，因此直接忽略插入结果
-                        _ = self.syscall.insert(co_id, coroutine);
-                        if let SyscallState::Suspend(timestamp) = state {
-                            self.syscall_suspend
-                                .push(SyscallSuspendItem { timestamp, co_id });
+                        if let SyscallState::Executing = state {
+                            //协程在系统调用执行期间被信号抢占，放回就绪队列以恢复执行
+                            self.ready.push(coroutine);
+                        } else {
+                            //挂起协程到系统调用表
+                            //如果已包含，说明当前系统调用还有上层父系统调用，因此直接忽略插入结果
+                            _ = self.syscall.insert(co_id, coroutine);
+                            if let SyscallState::Suspend(timestamp) = state {
+                                self.syscall_suspend
+                                    .push(SyscallSuspendItem { timestamp, co_id });
+                            }
                         }
                     }
                     CoroutineState::Suspend((), timestamp) => {