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-hnsw/leann_backend_hnsw/hnsw_backend.py

@@ -2,7 +2,7 @@ import logging
 import os
 import shutil
 from pathlib import Path
-from typing import Any, Literal
+from typing import Any, Literal, Optional
 
 import numpy as np
 from leann.interface import (
@@ -152,7 +152,7 @@ class HNSWSearcher(BaseSearcher):
         self,
         query: np.ndarray,
         top_k: int,
-        zmq_port: int | None = None,
+        zmq_port: Optional[int] = None,
         complexity: int = 64,
         beam_width: int = 1,
         prune_ratio: float = 0.0,