[tests] Update API tests to use /api prefix

- Simplified api_client fixture to hardcode /api prefix
- Updated websocket endpoint test to use /api/ws
- Fixed internal endpoint tests to not use /api prefix
- Enhanced validation.py to handle OpenAPI nullable syntax

tests-api/conftest.py

@@ -93,7 +93,8 @@ def api_client(base_url: str) -> Generator[Optional[requests.Session], None, Non
 
     # Helper function to construct URLs
     def get_url(path: str) -> str:
-        return urljoin(base_url, path)
+        # All API endpoints use the /api prefix
+        return urljoin(base_url, '/api' + path)
 
     # Add url helper to the session
     session.get_url = get_url  # type: ignore