fix: Python 3.9 compat — replace str | None with Optional[str] in serve.py

The `str | None` union syntax at module level requires Python 3.10+.
serve.py cannot use `from __future__ import annotations` because it
defines Pydantic models inside functions (FastAPI needs runtime types).

soup_cli/commands/serve.py

@@ -24,7 +24,7 @@ def _validate_adapter_name(name: str) -> bool:
     return bool(re.match(r'^[a-zA-Z0-9][a-zA-Z0-9\-]*$', name))
 
 
-def _validate_adapter_path(path: str, cwd: str | None = None) -> bool:
+def _validate_adapter_path(path: str, cwd: Optional[str] = None) -> bool:
     """Validate adapter path: must exist and stay under cwd."""
     if cwd is None:
         cwd = str(Path.cwd())