Function extract_token 

pub fn extract_token<R>(req: &R, token_name: &str) -> Option<String>
where
    R: SaRequest,

Generic token extraction from any SaRequest implementation. 从任意 SaRequest 实现中按统一顺序提取 Token。

Order | 顺序:

1. Header [token_name] (Bearer semantics via extract_bearer_or_value).
2. Authorization header if token_name is not already Authorization (case-insensitive match on read side is adapter-specific).
3. Cookie [token_name].
4. Query parameter [token_name].

Empty strings are skipped. Returns None if nothing found. 空字符串跳过；均未命中则返回 None。