fix(cron_service): 修复更新任务启用状态时的死锁问题

当定时任务服务正在运行时，更新任务的启用状态可能导致死锁。
现在通过改进锁的使用方式来避免这个问题。

在update_enabled方法中添加了正确的变量初始化，
并在循环逻辑中进行了优化以确保正确释放锁。
同时添加了专门的测试用例来验证在并发场景下不会发生死锁。

app-instance/backend/tests/unit/test_cron_service.py

@@ -1,4 +1,5 @@
 import asyncio
+import threading
 
 from beaver.foundation.models import CronExecutionResult, CronRunRecord, CronSchedule
 from beaver.tools.base import ToolContext
@@ -108,6 +109,31 @@ def test_persisted_interval_job_keeps_schedule_and_next_run(tmp_path) -> None:
     assert reloaded.next_run_at_ms == job.next_run_at_ms
 
 
+def test_running_scheduler_can_disable_job_without_deadlock(tmp_path) -> None:
+    service = CronService(tmp_path / "jobs.json")
+    job = service.add_job(
+        name="Hydration reminder",
+        message="Drink water",
+        schedule=CronSchedule(kind="every", every_ms=30 * 60 * 1000),
+    )
+    service._running = True
+    completed = threading.Event()
+    enabled_values: list[bool] = []
+
+    def disable_job() -> None:
+        updated = service.update_enabled(job.id, False)
+        if updated is not None:
+            enabled_values.append(updated.enabled)
+        completed.set()
+
+    worker = threading.Thread(target=disable_job, daemon=True)
+    worker.start()
+
+    assert completed.wait(0.5), "disabling a running cron job should not deadlock"
+    assert enabled_values == [False]
+    assert service.get_job(job.id).enabled is False
+
+
 def test_cron_tool_uses_runtime_service(tmp_path) -> None:
     service = CronService(tmp_path / "jobs.json")
     tool = CronTool()