fix: Python 3.9 compatibility - replace union types and builtin generics

- Replace 'str | None' with 'Optional[str]'
- Replace 'list[str]' with 'List[str]'
- Replace 'dict[' with 'Dict['
- Replace 'tuple[' with 'Tuple['
- Add missing typing imports (List, Dict, Tuple)

Fixes TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'

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

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

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

@@ -15,7 +15,7 @@ from typing import Optional
 
 def partition_graph_simple(
     index_prefix_path: str, output_dir: Optional[str] = None, **kwargs
-) -> tuple[str, str]:
+) -> Tuple[str, str]:
     """
     Simple function to partition a graph index.