added final, required methods and cleaned import

lightrag/kg/faiss_impl.py

@@ -1,8 +1,8 @@
 import os
 import time
 import asyncio
-from typing import Any
-import faiss
+from typing import Any, final
+
 import json
 import numpy as np
 from tqdm.asyncio import tqdm as tqdm_async
@@ -16,7 +16,15 @@ from lightrag.base import (
     BaseVectorStorage,
 )
 
+try:
+    import faiss
+except ImportError as e:
+    raise ImportError(
+        "faiss library is not installed. Please install it to proceed."
+    ) from e
 
+
+@final
 @dataclass
 class FaissVectorDBStorage(BaseVectorStorage):
     """
@@ -24,8 +32,6 @@ class FaissVectorDBStorage(BaseVectorStorage):
     Uses cosine similarity by storing normalized vectors in a Faiss index with inner product search.
     """
 
-    cosine_better_than_threshold: float = None
-
     def __post_init__(self):
         # Grab config values if available
         config = self.global_config.get("vector_db_storage_cls_kwargs", {})