fix(knowledge): list_collections 增加异常保护，防止 ChromaDB 数据丢失导致 /collections 500

遍历 kb_metadata.json 时对每个集合的 get_collection/count 操作包裹
try/except，捕获 NotFoundError/InternalError 等异常后跳过并记录警告，
循环结束后自动清理失效的元数据条目。

knowledge/collection.py

@@ -382,16 +382,31 @@ class CollectionMixin:
 
         self._save_metadata()
 
+        stale_collections = []
+
         for name, info in self._metadata.get("collections", {}).items():
-            collection = self.get_collection(name)
-            result.append(CollectionInfo(
-                name=name,
-                display_name=info.get("display_name", name),
-                document_count=collection.count() if collection else 0,
-                created_at=info.get("created_at", ""),
-                department=info.get("department", ""),
-                description=info.get("description", "")
-            ))
+            try:
+                collection = self.get_collection(name)
+                result.append(CollectionInfo(
+                    name=name,
+                    display_name=info.get("display_name", name),
+                    document_count=collection.count() if collection else 0,
+                    created_at=info.get("created_at", ""),
+                    department=info.get("department", ""),
+                    description=info.get("description", "")
+                ))
+            except Exception as e:
+                logger.warning(
+                    f"跳过异常向量库 '{name}': {e}，可能是 ChromaDB 数据目录已丢失"
+                )
+                stale_collections.append(name)
+
+        # 清理元数据中指向已失效集合的条目
+        if stale_collections:
+            for name in stale_collections:
+                self._metadata.get("collections", {}).pop(name, None)
+                logger.info(f"清理失效向量库元数据: {name}")
+            self._save_metadata()
 
         return result