Initialize token variable in A2AAuthMiddleware

Added explicit initialization of the 'token' variable to None in the _extract_token method to ensure it is always defined before use.

Author: loveyana

veadk/a2a/ve_middlewares.py

@@ -206,6 +206,8 @@ def _extract_token(self, request: Request) -> tuple[Optional[str], bool]:
             The extracted token, or None if not found
         """
         has_prefix = False
+        token = None
+        
         if self.auth_method == "header":
             # Extract from Authorization header
             auth_header = request.headers.get("Authorization") or request.headers.get(