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-hnsw/leann_backend_hnsw/hnsw_backend.py

@@ -61,7 +61,7 @@ class HNSWBuilder(LeannBackendBuilderInterface):
                     "is_recompute is False, but is_compact is True. This is not compatible now. change is compact to False and you can use the original HNSW index."
                 )
 
-    def build(self, data: np.ndarray, ids: list[str], index_path: str, **kwargs):
+    def build(self, data: np.ndarray, ids: List[str], index_path: str, **kwargs):
         from . import faiss  # type: ignore
 
         path = Path(index_path)
@@ -160,7 +160,7 @@ class HNSWSearcher(BaseSearcher):
         pruning_strategy: Literal["global", "local", "proportional"] = "global",
         batch_size: int = 0,
         **kwargs,
-    ) -> dict[str, Any]:
+    ) -> Dict[str, Any]:
         """
         Search for nearest neighbors using HNSW index.