fix(chat): 修复在海马体未初始化时调用主题遗忘的潜在错误

在调用 `operation_forget_topic` 方法前增加空值检查，以防止当海马体或其相关组件尚未完全初始化时，调用该方法导致程序崩溃。此更改增强了记忆遗忘操作的健壮性。

src/chat/memory_system/Hippocampus.py

@@ -1689,7 +1689,11 @@ class HippocampusManager:
         if not self._initialized:
             raise RuntimeError("HippocampusManager 尚未初始化，请先调用 initialize 方法")
         async with self._db_lock:
-            return await self._hippocampus.parahippocampal_gyrus.operation_forget_topic(percentage)
+            if self._hippocampus and self._hippocampus.parahippocampal_gyrus:
+                return await self._hippocampus.parahippocampal_gyrus.operation_forget_topic(percentage)
+            else:
+                logger.warning("海马体或海马旁回未初始化，跳过本次记忆遗忘。")
+                return None
 
     async def consolidate_memory(self):
         """整合记忆的公共接口"""