refactor(executors): 重构执行器逻辑，移除重复mixin并优化分层排序

主要变更：
1.  将任务跳过/重试逻辑从类mixin改为模块级函数，减少代码重复
2.  优化_graph.layers()的前置校验逻辑，统一在run入口执行
3.  重构存储过期检查API，移除废弃的_expired方法
4.  优化TaskSpec.cache_key异常处理，增加指定异常捕获并记录警告
5.  修复verbose模式下的事件回调逻辑，正确触发RUNNING事件
6.  调整测试用例以适配新的API和行为变更

tests/test_storage.py

@@ -70,9 +70,9 @@ def test_memory_backend_ttl_load_filters_expired() -> None:
 
 
 def test_memory_backend_expired_key_not_in_store() -> None:
-    """_expired 对不存在键返回 False."""
+    """不存在的键 has 返回 False."""
     b = MemoryBackend(ttl=1.0)
-    assert b._expired("nonexistent") is False
+    assert b.has("nonexistent") is False
 
 
 def test_memory_backend_no_ttl_never_expired() -> None:
@@ -244,35 +244,35 @@ def test_json_backend_ttl_load_filters_expired() -> None:
 
 
 def test_json_backend_expired_no_ttl() -> None:
-    """无 TTL 时 _expired 返回 False."""
+    """无 TTL 时永不过期."""
     with tempfile.TemporaryDirectory() as tmp:
         path = str(Path(tmp) / "state.json")
         b = JSONBackend(path)
         b.save("a", 1)
         # 手动修改 ts 为很久以前
         b._store["a"]["ts"] = time.time() - 1000
-        assert b._expired(b._store["a"]) is False  # 无 TTL，永不过期
+        assert b.has("a") is True  # 无 TTL，永不过期
 
 
 def test_json_backend_expired_with_ttl() -> None:
-    """有 TTL 时 _expired 检查是否过期."""
+    """有 TTL 时过期键 has 返回 False."""
     with tempfile.TemporaryDirectory() as tmp:
         path = str(Path(tmp) / "state.json")
         b = JSONBackend(path, ttl=1.0)
         b.save("a", 1)
         # 手动修改 ts 为很久以前
         b._store["a"]["ts"] = time.time() - 10  # 10 秒前，超过 TTL
-        assert b._expired(b._store["a"]) is True
+        assert b.has("a") is False
 
 
 def test_json_backend_expired_missing_ts() -> None:
-    """entry 缺少 ts 时使用默认值 0."""
+    """entry 缺少 ts 时视为过期."""
     with tempfile.TemporaryDirectory() as tmp:
         path = str(Path(tmp) / "state.json")
         b = JSONBackend(path, ttl=1.0)
         b._store["a"] = {"value": 1}  # 缺少 ts
         # ts 默认为 0，已经过了很久
-        assert b._expired(b._store["a"]) is True
+        assert b.has("a") is False
 
 
 def test_json_backend_save_value_error(monkeypatch: pytest.MonkeyPatch) -> None: