fix: Python 3.9 compatibility - replace Union type syntax

- Replace 'int | None' with 'Optional[int]' everywhere
- Replace 'subprocess.Popen | None' with 'Optional[subprocess.Popen]'
- Add Optional import to all affected files
- Update ruff target-version from py310 to py39
- The '|' syntax for Union types was introduced in Python 3.10 (PEP 604)

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

pyproject.toml

@@ -89,7 +89,7 @@ leann-backend-diskann = { path = "packages/leann-backend-diskann", editable = tr
 leann-backend-hnsw = { path = "packages/leann-backend-hnsw", editable = true }
 
 [tool.ruff]
-target-version = "py310"
+target-version = "py39"
 line-length = 100
 extend-exclude = [
     "third_party",