fix: drain cancelled debounce tasks

tests/test_debounce.py

@@ -0,0 +1,59 @@
+"""同步与异步防抖生命周期测试。"""
+
+import asyncio
+
+import pytest
+
+from app.runtime.debounce import AsyncDebouncer
+
+
+@pytest.mark.asyncio
+async def test_async_debouncer_cancel_waits_for_replaced_task_cleanup() -> None:
+    """公开取消必须等待已被后续调用替换的任务完成取消清理。"""
+    first_started = asyncio.Event()
+    first_cancelled = asyncio.Event()
+    allow_first_cleanup = asyncio.Event()
+
+    async def delayed_work(value: str) -> None:
+        """让首个调用在取消后停留，暴露 retired task 的终态时序。"""
+        if value != "first":
+            await asyncio.Event().wait()
+            return
+        first_started.set()
+        try:
+            await asyncio.Event().wait()
+        except asyncio.CancelledError:
+            first_cancelled.set()
+            await allow_first_cleanup.wait()
+            raise
+
+    debouncer = AsyncDebouncer(delayed_work, interval=0)
+    await debouncer("first")
+    await first_started.wait()
+
+    await debouncer("second")
+    await first_cancelled.wait()
+    cancel_task = asyncio.create_task(debouncer.cancel())
+    await asyncio.sleep(0)
+
+    assert cancel_task.done() is False
+    allow_first_cleanup.set()
+    await asyncio.wait_for(cancel_task, timeout=1)
+    assert debouncer.task is None
+    assert debouncer._retired_tasks == set()
+
+
+@pytest.mark.asyncio
+async def test_async_debouncer_cancel_is_idempotent_without_active_task() -> None:
+    """没有活动任务时重复取消应立即返回并保持冷却状态关闭。"""
+
+    async def no_op() -> None:
+        """提供不会被实际调度的异步函数。"""
+
+    debouncer = AsyncDebouncer(no_op, interval=1, leading=True)
+
+    await debouncer.cancel()
+    await debouncer.cancel()
+
+    assert debouncer.task is None
+    assert debouncer.is_cooling_down is False