style: organize imports per ruff; finish py39 Optional changes

- Fix import ordering in embedding servers and graph_partition_simple
- Remove duplicate Optional import
- Complete Optional[...] replacements

packages/leann-backend-diskann/leann_backend_diskann/diskann_embedding_server.py

@@ -10,6 +10,7 @@ import sys
 import threading
 import time
 from pathlib import Path
+from typing import Optional
 
 import numpy as np
 import zmq
@@ -32,7 +33,7 @@ if not logger.handlers:
 
 
 def create_diskann_embedding_server(
-    passages_file: str | None = None,
+    passages_file: Optional[str] = None,
     zmq_port: int = 5555,
     model_name: str = "sentence-transformers/all-mpnet-base-v2",
     embedding_mode: str = "sentence-transformers",

packages/leann-backend-diskann/leann_backend_diskann/graph_partition.py

@@ -12,6 +12,7 @@ import shutil
 import subprocess
 import tempfile
 from pathlib import Path
+from typing import Optional
 
 
 class GraphPartitioner:
@@ -88,8 +89,8 @@ class GraphPartitioner:
     def partition_graph(
         self,
         index_prefix_path: str,
-        output_dir: str | None = None,
-        partition_prefix: str | None = None,
+        output_dir: Optional[str] = None,
+        partition_prefix: Optional[str] = None,
         **kwargs,
     ) -> tuple[str, str]:
         """

packages/leann-backend-diskann/leann_backend_diskann/graph_partition_simple.py

@@ -10,10 +10,11 @@ import os
 import subprocess
 import tempfile
 from pathlib import Path
+from typing import Optional
 
 
 def partition_graph_simple(
-    index_prefix_path: str, output_dir: str | None = None, **kwargs
+    index_prefix_path: str, output_dir: Optional[str] = None, **kwargs
 ) -> tuple[str, str]:
     """
     Simple function to partition a graph index.