fix: complete Python 3.9 type annotation fixes in backend packages

Fix remaining Python 3.9 incompatible type annotations in backend packages
that were causing test failures. The union operator (|) syntax for type hints
was introduced in Python 3.10 and causes "TypeError: unsupported operand
type(s) for |" errors in Python 3.9.

Changes in leann-backend-diskann:
- Convert zmq_port: int | None to Optional[int] in diskann_backend.py
- Convert passages_file: str | None to Optional[str] in diskann_embedding_server.py
- Add Optional imports to both files

Changes in leann-backend-hnsw:
- Convert zmq_port: int | None to Optional[int] in hnsw_backend.py
- Add Optional import

This resolves the final test failures related to type annotation syntax and
ensures full Python 3.9 compatibility across all packages.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

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",