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_backend.py

@@ -4,7 +4,7 @@ import os
 import struct
 import sys
 from pathlib import Path
-from typing import Any, Literal
+from typing import Any, Literal, Optional
 
 import numpy as np
 import psutil
@@ -259,7 +259,7 @@ class DiskannSearcher(BaseSearcher):
         prune_ratio: float = 0.0,
         recompute_embeddings: bool = False,
         pruning_strategy: Literal["global", "local", "proportional"] = "global",
-        zmq_port: int | None = None,
+        zmq_port: Optional[int] = None,
         batch_recompute: bool = False,
         dedup_node_dis: bool = False,
         **kwargs,